\ovrline and \undeline are not macros so they can’t be redefined with \pend:defI.
<..plain math..>+
\def\:temp#1{\relax \ifvmode\leavevmode\fi
\a:underline\hbox{$#1$}\b:underline}
\HLet\underline=\:temp
\NewConfigure{underline}{2}
-_-_-
Some environments call \@@underline directly, so we want this macro also to be defined directly. However, this macro always appears within math mode, so deine also \underline directly.
<..latex math..>+
\def\underline#1{%
\relax
\ifmmode\o:@@underline:{#1}%
\else $\o:@@underline:{\hbox{#1}}\m@th$\relax\fi}
\def\:tempc#1{\relax\ifvmode\leavevmode\fi
\a:underline \o:@@underline:{#1}\b:underline}
\HLet\@@underline=\:tempc
\def\:tempc#1{\relax\ifvmode\leavevmode\fi
\a:underline \o:underline:{#1}\b:underline}
\HLet\underline=\:tempc
\NewConfigure{underline}{2}
-_-_-
<..plain,latex math..>+
\def\:temp#1{\relax\ifvmode\leavevmode\fi
\a:overline\hbox{$<.empty base for sub/sup.>#1$}\b:overline}
\HLet\overline=\:temp
\NewConfigure{overline}{2}
-_-_-
<..empty base for sub/sup..>
{\HCode{}}-_-_-