\documentclass{ltjarticle}
\usepackage[OT1]{fontenc}
\DeclareSymbolFont{hogemath}{OT1}{hoge}{m}{n}
\SetSymbolFont{hogemath}{bold}{OT1}{hoge}{bx}{n}
\DeclareSymbolFont{piyomath}{JY3}{piyo}{m}{n}
\SetSymbolFont{piyomath}{bold}{JY3}{piyo}{bx}{n}
%
\DeclareFontFamily{OT1}{guha}{}
\DeclareFontShape{OT1}{guha}{m}{n}{<-> cmfi10}{}
\DeclareFontShape{JY3}{gt}{l}{it}{<-> file:HaranoAjiMincho-Heavy.otf:jfm=ujis}{}
\DeclareFontSubstitution{OT1}{guha}{m}{n}
\DeclareKanjiSubstitution{JY3}{gt}{l}{it}
%
\begin{document}
\romanfamily{fuga}\kanjifamily{fuga}\selectfont abcあいうえお
\setbox0=\hbox{$\empty$}%
\end{document}
