Here is a plain tex example, not quite as minimal as I'd like but out
of time for now.
This hyphenates with pdftex but not xetex
\nopagenumbers
\catcode`\@=11
\ifx\Umathchar\@undefined
\font\x=cmr12
\else
\font\x="[lmroman12-regular]:mapping=tex-text"
\fi
\x
\newbox\FN@tempboxa
\newbox\FN@tempboxb
\newbox\FN@tempboxc
\long\def\makefootnoteparagraph{\unvbox0 \makehboxofhboxes
\setbox\FN@tempboxa=\hbox{\unhbox\FN@tempboxa \removehboxes}
\noindent
\unhbox\FN@tempboxa\par
}
\def\makehboxofhboxes{\setbox\FN@tempboxa=\hbox{}%
\loop
\setbox\FN@tempboxb=\lastbox
\ifhbox\FN@tempboxb
\setbox\FN@tempboxa=\hbox{\box\FN@tempboxb\unhbox\FN@tempboxa}%
\repeat
}
\def\removehboxes{\setbox\FN@tempboxa=\lastbox
\ifhbox
\FN@tempboxa{\removehboxes}%
\unhbox\FN@tempboxa
\fi
}
\setbox0\vbox{XXXXXXXXXX just a few normal words to fill up the line
up to my x x x zzzzzz\-zzzzz}
\global\setbox\FN@tempboxc\vbox{\hbox{XXX}\makefootnoteparagraph}%
\tracingonline1\showbox\FN@tempboxc
\unvbox\FN@tempboxc
\bye
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