In the LaTeX template <R-Library>/rmarkdown/rmd/latex/default-1.17.0.2.tex we see
\begin{document}
$if(title)$
\maketitle
$endif$
$if(abstract)$
\begin{abstract}
$abstract$
\end{abstract}
$endif$
$for(include-before)$
$include-before$
This means that a titlepage is created using \maketitle if a title is defined in the YAML headers. Similar for the abstract. If you remove both these tags from your YAML headers, then the content from the file body.tex will be the first to be processed and you are free to customize your titlepage there.
See the answers to this question for an alternative approach.