I needed to do this today and found your question but no answer. This is what I finally came up with. You can do this by setting up a function in the script editor named getNote.
function getNote(cell)
{
var ss = SpreadsheetApp.getActiveSpreadsheet();
var range = ss.getRange(cell)
return range.getNote();
}
then in your spreadsheet enter:
=getNote("C4")
to get the note from cell C4. If you want it to be a reference that changes when you move around the cells, then reference it this way:
=getNote(Address(row(C4), column(C4)))
Word of caution: Notes don't trigger a refresh, so if a note is edited the cell does not pick up the new value right away