Hi!

I rotated this in my head overnight, and have decided that the
least-worst solution compatibility-wise is to do
  .if !ddoc-Ar-font .ds doc-Ar-font \fI
  .if !ddoc-Li-font .ds doc-Li-font \f(CR
  .if !ddoc-Nm-font .ds doc-Nm-font \fB
  .if !ddoc-Pa-font .ds doc-Pa-font \fI
&c., which is correct for groff mdoc (OOB)
and mandoc (by font-matching the default styling);
with a custom style, this can trivially be made to match by injecting
  printf '.ds doc-%s-font \\f[%s]\n' Ar CI  Nm CB  [macro font]...
for -Thtml output, since that's where you specify -Ostyle anyway.

Thanks for your input on this :)

Best,
наб