I tested the following file

============================================ \let\normalfont\font \def\font#1=#2 {% \message{Adding mapline: +#2 <#2.pfb}% \pdfmapline{+#2 <#2.pfb}\normalfont#1=#2 }

\pdfoutput=1 \font\f=cmx10 at 100pt \f Hello \bye ============================================

and it seems to be ok. What error did you get?

It is ok here also. I have erased cmr10 from the pdftex.map and tested by \font\f=cmr10 at 100pt. Best regards Akira