With pdftex it is possible to restrict a \pdfglyphtounicode
declaration to a specific tfm. So in the following example the A
from cmr10 is mapped to B, but the A from cmss10 is untouched:

\pdfgentounicode=1
\pdfglyphtounicode{tfm:cmr10/A}{0042}
A \font\test=cmss10 \test  A
\bye

This is useful, e.g., to setup unicode mappings for older symbol
fonts which often (mis)use standard glyph names.

With luatex this doesn't work. Only the "general" mapping is honored
here:

\pdfvariable gentounicode =1
\pdfextension glyphtounicode {C}{0044}
\pdfextension glyphtounicode {tfm:cmr10/A}{0042}

C A \font\test=cmss10 \test C A
\bye

Would it be possible to extend luatex to support the tfm: syntax
too? Or is there an alternative to change tounicode mappings?