3
There's \valign. Enjoy and don't ask.
\documentclass{article}
\newcommand{\halfscript}[2]{%
\mathord{\hbox{% ensure math mode
\valign{%
\vfil##\vfil\cr
\hbox{$#1$}\cr
\hbox{$\scriptstyle#2$}\cr
}%
\kern\scriptspace
}}%
}
\newcommand{\prehalfscript}[2]{%
\math...