Handles accents that reach \add@accent. Currently just empty bases?
<..latex ltoutenc..>+
\def\:tempc#1#2{\expandafter
\ifx \csname accent \cf@encoding :#1\endcsname\relax
\o:add@accent:{#1}{#2}%
\else \def\:temp{\add:accent{#1}{#2}}%
\expandafter\expandafter\expandafter\:temp
\csname accent \cf@encoding :#1\endcsname{}{}<.par del.>\fi }
\HLet\add@accent\:tempc
\def\add:accent#1#2#3#4{%
\def\:temp{#3#4}\ifx \:temp\empty
\o:add@accent:{#1}{#2}%
\expandafter\expandafter\expandafter\gob:pardel
\else \def\:temp{#2}\def\:tempa{#3}\ifx \:temp\:tempa
#4\expandafter\expandafter\expandafter\gob:pardel
\else
\expand:after{\expand:after{\add:accent{#1}{#2}}}%
\fi \fi }
\def\gob:pardel#1<.par del.>{}
-_-_-
<..latex ltoutenc..>+
\NewConfigure{add accent}[3]{%
\expandafter\ifx \csname accent #1\endcsname\relax
\expandafter\let\csname accent #1\endcsname=\empty
\else \def\:temp{#2#3}\ifx \:temp\empty
\expandafter\let\csname accent #1\endcsname=\empty
\fi \fi
\expandafter\scan:accents\csname accent #1\endcsname{#2}{#3}%
}
\def\scan:accents#1#2#3{\def\:temp{#2#3}\ifx \:temp\empty\else
\append:def#1{{#2}{#3}}\expand:after{\scan:accents#1}%
\fi}
-_-_-
The first argument should be an encoding:accent-number pair.