The following redefinition of \AM@checklast allows you to now use the new key prelast to insert up to the before-last page:
\usepackage{pdfpages}% http://ctan.org/pkg/pdfpages
\makeatletter
\def\AM@checklast#1-\END{%
\edef\AM@tempii{\AM@trim@space{#1}}%
\expandafter\ifx\expandafter\\\AM@tempii\\
\@tempcntb=\AM@pagecount\relax
\ifAM@DVIoutput
\ifnum\AM@pagecount=1
\@tempcntb\@tempcnta
\fi
\fi
\advance\@tempcntb\@ne
\whiledo{\@tempcnta<\@tempcntb}{%
\AM@append{\the\@tempcnta}%
\advance\@tempcnta\@ne
}%
\else
\def\AM@cmp{last}%
\ifx\AM@cmp\AM@tempii
\let\AM@tempii\AM@pagecount
\else\def\AM@cmp{prelast}% <------------------------- ADDED
\ifx\AM@cmp\AM@tempii% |
\@tempcntb=\numexpr\number\AM@pagecount-1\relax% |
\edef\AM@tempii{\the\@tempcntb}% |
\fi% <---------------------------------------------------
\fi
\AM@checkinteger{\AM@tempii}%
\ifAM@integer
\@tempcntb=\AM@tempii\relax
\AM@checkpagenumber{\the\@tempcntb}%
\ifnum\@tempcnta<\@tempcntb \def\AM@rel{<}\def\AM@inc{\@ne}%
\else \def\AM@rel{>}\def\AM@inc{\m@ne}\fi
\advance\@tempcntb\AM@inc
\whiledo{\@tempcnta\AM@rel\@tempcntb}{%
\AM@append{\the\@tempcnta}%
\advance\@tempcnta\AM@inc
}%
\else
\AM@setnewdocname{\AM@temp}%
\fi
\fi
}
\makeatother
The idea behind it is to insert a test case for the prelast "page", and to subtract one from \AM@pagecount - the number of pages within the document being read.
The intended use would be
\includepdf[pages=16-prelast]{file}