I am using a 'shares model' to estimate values for missing observations.  With the example data set my.data I am filling missing observations for each of three years proportionally to how observations were distributed in 1970 (although I could do so using 2010 or both 1970 and 2010).  
Below I present example data, the desired result and code to obtain desired estimates in two ways. Code for the first approach is very specific to this example. I wish to create a more-general function than that used in the second approach. Creating a more-general function seems to me to require calling a function on a list of lists. I am hoping someone might offer advice on how to apply a function to a list of lists.
Here is the example data set and the highly specific solution:
my.data <- read.table(text = '
 county  y1970  y1980  y1990  y2000  y2010
   aa      50     NA     70     NA     500
   cc      10     20     NA     NA     100
   ee     800     NA     NA    400    8000
   gg    1000   1900     NA     NA   10000
   ii     200    400    300    100    2000
   kk      20     40     30     NA     200
', header = TRUE, na.string='NA', stringsAsFactors=FALSE)
my.total <- read.table(text = '
   county  y1970  y1980  y1990  y2000  y2010
   total    2080   4000   3000   1000  20800
', header = TRUE, na.string='NA', stringsAsFactors=FALSE)
desired.result <- read.table(text = '
 county  y1970  y1980         y1990       y2000       y2010
   aa      50  96.47059         70      23.148148       500
   cc      10     20         14.36464    4.629630       100
   ee     800   1543.529   1149.17127      400         8000
   gg    1000   1900       1436.46409   462.962963    10000
   ii     200    400           300         100         2000
   kk      20     40            30       9.259259       200
', header = TRUE, na.string='NA', stringsAsFactors=FALSE)
x70 <- c(50, 800)
estimates.for.80 <- (x70 / sum(x70)) * (my.total$y1980 - sum(my.data$y1980, na.rm = TRUE))
x80 <- c(10, 800, 1000)
estimates.for.90 <- (x80 / sum(x80)) * (my.total$y1990 - sum(my.data$y1990, na.rm = TRUE))
x90 <- c(50, 10, 1000, 20)
estimates.for.00 <- (x90 / sum(x90)) * (my.total$y2000 - sum(my.data$y2000, na.rm = TRUE))
Here is the function.  I think this can be generalized if I knew how to include d.counties as an input list to the function.  In other words, how can I include d.counties in my.input and still have the function work?  My confusion I think stems from the length of d.counties differing among years.
state <- 'my.state'
my.df <- read.table(text = '
  county   y1970  y1980  y1990  y2000   y2010
      aa      50     NA     70     NA     500
      cc      10     20     NA     NA     100
      ee     800     NA     NA    400    8000
      gg    1000   1900     NA     NA   10000
      ii     200    400    300    100    2000
      kk      20     40     30     NA     200
   total    2080   4000   3000   1000   20800
', header = TRUE, na.string='NA', stringsAsFactors=FALSE)
pre.divide.up <- tail(my.df[,2:ncol(my.df)], 1) - colSums(head(my.df[,2:ncol(my.df)], -1), na.rm = TRUE)
# For each column containing NA's define the years to use as shares
# If use.years = 'pre'  then use the year in pre.year
# If use.years = 'post' then use the year in post.year
# If use.years = 'both' then use both the year in pre.year and the year in post.year
#
# Here I define pre.year = y1970 and post.year = 2010 for every year
# However, 'pre.year' and 'post.year' are variables.  They can differ among rows below.
shares <- read.table(text = '
      cyear   pre.year  post.year  use.years
      y1980     y1970     y2010        pre
      y1990     y1970     y2010        pre
      y2000     y1970     y2010        pre
', header = TRUE, na.strings = "NA")
d.counties.80 <- c( 'aa' ,
                    'ee' )
d.counties.90 <- c( 'cc' ,
                    'ee' , 
                    'gg' )
d.counties.00 <- c( 'aa' ,
                    'cc' ,
                    'gg' ,
                    'kk' )
d.counties <- list(d.counties.80, d.counties.90, d.counties.00)
my.input <- data.frame(shares)
my.function <- function(y) {
# extract years of interest from my.df and store in data.frame called year.data
if(y[[4]] != 'last') year.data = my.df[names(my.df) %in% c("county", y[[2]], y[[1]], y[[3]])]
if(y[[4]] == 'last') year.data = my.df[names(my.df) %in% c("county", y[[2]], y[[1]]        )]
# subset counties in year.data to only include counties with NA's in current year
if(as.numeric(substr(y[1], 2, 5)) == 1980) year.data = year.data[year.data$county %in% d.counties.80,]
if(as.numeric(substr(y[1], 2, 5)) == 1990) year.data = year.data[year.data$county %in% d.counties.90,]
if(as.numeric(substr(y[1], 2, 5)) == 2000) year.data = year.data[year.data$county %in% d.counties.00,]
# reorder columns in year.data
if(y[[4]] != 'last') year.data = year.data[, c('county', y[[2]], y[[1]], y[[3]])]
if(y[[4]] == 'last') year.data = year.data[, c('county', y[[2]], y[[1]]        )]
# values to be divided, or distributed, among counties with NA's in the current year
divide.up <- pre.divide.up[, y[[1]]] 
# sum values from designated pre and/or post years and bind those totals to bottom of year.data
if(y[[4]] != 'last') colsums.year = data.frame('total', as.data.frame(t(as.numeric(colSums(year.data[,c(2:4)], na.rm=TRUE)))))
if(y[[4]] == 'last') colsums.year = data.frame('total', as.data.frame(t(as.numeric(colSums(year.data[,c(2:3)], na.rm=TRUE)))))
names(colsums.year) <- names(year.data)
year.data.b <- rbind(year.data, colsums.year)
# obtain percentages in designated pre and/or post years for counties with NA's in current year
year.data.c <- year.data.b
year.data.c[, -1] <- lapply( year.data.c[  , -1], function(x){ x/x[nrow(year.data.b)] } )
# estimate county values for current year by distributing total missing values in current year
# according to how values were distributed in those same counties in other years
if(y[[4]] == 'both') year.data.b[, y[[1]]] = rowMeans(data.frame(year.data.c[, y[[2]]], year.data.c[, y[[3]]])) * as.numeric(divide.up)
if(y[[4]] ==  'pre') year.data.b[, y[[1]]] = year.data.c[, y[[2]]] * as.numeric(divide.up)
if(y[[4]] == 'post') year.data.b[, y[[1]]] = year.data.c[, y[[3]]] * as.numeric(divide.up)
if(y[[4]] == 'last') year.data.b[, y[[1]]] = year.data.c[, y[[2]]] * as.numeric(divide.up)
# extract estimates for current year along with the county column, then remove the last row
year.data.last <- year.data.b[names(year.data.b) %in% c("county", y[[1]])]
year.data.last <- year.data.last[-nrow(year.data.last),]
colnames(year.data.last) <- c('county', 'acreage')
# create a data set for export
this.year <- rep(as.numeric(substr(y[[1]], 2, 5)), nrow(year.data.last))
revised.data <- data.frame(state, this.year, year.data.last)
return(revised.data) 
}
my.list  <- apply(shares, 1, function(y) my.function(y))
my.list2 <- do.call("rbind", my.list)
my.list2
      state this.year county     acreage
1  my.state      1980     aa   96.470588
3  my.state      1980     ee 1543.529412
2  my.state      1990     cc   14.364641
31 my.state      1990     ee 1149.171271
4  my.state      1990     gg 1436.464088
11 my.state      2000     aa   23.148148
21 my.state      2000     cc    4.629630
41 my.state      2000     gg  462.962963
6  my.state      2000     kk    9.259259
Although this function is not as general as the one in my answer below, the function above does allow explicit designation of which counties have relevant missing values. In the actual data there are two types of missing values and the function in my answer below cannot tell the two types apart. The function above can tell them apart because I tell it exactly which counties to consider each year.
Thank you again for any advice and for advice already offered.
 
     
    