pLaTeX の \fontfamily で .fd を探索する件

ot1piyo.fd

\DeclareFontFamily{OT1}{piyo}{}
\DeclareFontShape{OT1}{piyo}{m}{n}{<->cmfi10}{}
\endinput

testfd.tex

\documentclass{jarticle}
\usepackage[T1,TS1,OT1]{fontenc}
\def\TEST#1{%
\paragraph{#1}
\fontencoding{#1}\selectfont
ほげThe quick brown fox jumps over the lazy dog.
\edef\M{\fontname\font}\texttt{\fontencoding{OT1}\selectfont\M}

{\fontfamily{piyo}\selectfont
ほげThe quick brown fox jumps over the lazy dog.
\edef\M{\fontname\font}\texttt{\fontencoding{OT1}\selectfont\M}
}\par}

\begin{document}
\TEST{T1}\TEST{OT1}\TEST{TS1}
\end{document}