\documentclass{article}
\textheight.3\textheight
\paperheight.5\paperheight
\pdfpageheight\paperheight

\usepackage{longtable}
\makeatletter
\def\foopage#1#2{\global\expandafter\let\csname foopage-#1-#2\endcsname\@empty}

\let\foolist\@empty
\def\foodef#1#2{%
\g@addto@macro\foolist{{#1}}%
\@namedef{foo-#1}{#2}}

\def\foocite#1{%
\textsuperscript{#1}%
\let\reserved@a\relax
\protected@write\@auxout{}{\string\foopage{#1}{\thepage}}%
}

\def\fooloop{%
\expandafter\@tfor\expandafter\tmp\expandafter:\expandafter=\foolist\do{%
\expandafter\ifx\csname foopage-\tmp-\thepage\endcsname\@empty
\insert\footins{%
    \reset@font\footnotesize
    \interlinepenalty\interfootnotelinepenalty
    \splittopskip\footnotesep
    \splitmaxdepth \dp\strutbox \floatingpenalty \@MM
    \hsize\columnwidth \@parboxrestore
    \textsuperscript{\tmp}\csname foo-\tmp\endcsname}%
\fi
}}


% add the insert loop
\def\foopatch#1\output#2{%
\def\LT@start{#1\fooloop\output{\LT@output}}}
\expandafter\foopatch\LT@start


\def\foopatch#1\copy\LT@head\nobreak#2{%
\def\LT@output{#1\copy\LT@head\nobreak\fooloop#2}}
\expandafter\foopatch\LT@output


\begin{document}


\foodef{A}{Something about A.}
\foodef{i}{Something equally dull about i.}
\foodef{w}{In case you missed $x$ $y$ and $z$, here's $w$. I repeat, here's $w$. Actually, forget about $y$, $w$ seems to be the key. And, to tell You the truth, I don't really like $x$. }


\begin{longtable}{ll}
aaaa&bbb\\
aaaa&bbb\\
aaaa&bbb\foocite{A}\\
aaaa&bbb\\
aaaa&bbb\\
aaaa&bbb\foocite{i}\\
aaaa&bbb\\
aaaa&bbb\\
aaaa&bbb\\
aaaa&bbb\\
aaaa&bbb\foocite{w}\\
aaaa&bbb\\
aaaa&bbb\\
aaaa&bbb\\
aaaa&bbb\\
aaaa&bbb\\
aaaa&bbb\\
\end{longtable}


\end{document}