Plain.tex doesn't allow two accents. You need something like this:
% file diatop.sty
% I use Christina Thiele's useful macro diatop.sty, which was published in
% her paper in the proceedings of the 1987 TeX Users Group. Here it is:
%
\def\diatop[#1|#2]{\leavevmode{\setbox1=\hbox{{#1{}}}\setbox2=\hbox{{#2{}}}%
\dimen0=\ifdim\wd1>\wd2\wd1\else\wd2\fi%
\dimen1=\ht2\advance\dimen1by-1ex%
\setbox1=\hbox to1\dimen0{\hss#1\hss}%
\rlap{\raise1\dimen1\box1}%
\hbox to1\dimen0{\hss#2\hss}}}%
\endinput
%
%E.g. of use: \diatop[\'|{\=o}] gives u macron acute
%
%From {\em TeX Users Group Eighth Annual: Conference Proceedings}
%Meeting, Seattle, August 24-26, 1987, TeXniques no. 5, (Providence,
%1988), p.11.
%
%Dominik
PS, this doesn't address the empty glyph issue that you raised originally.
Dominik Wujastyk
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