Conditional "Suggests"

To me this reads as being in the wheelhouse of what 'Suggests' is supposed
to imply, as per the R Extensions manual. ?The problem here is that if PkgB
is put down as 'Suggests', it is required for R CMD check to pass which
seems to defeat the purpose of this exercise a bit.

No, because R CMD check is only run by the developer, not the user.

It's not only run by the developer, it's also run by CRAN.  The situation 
might happen where for a particular platform (e.g. Windows) that PkgB 
doesn't exist.  It'd be nice to be able to avoid a situation where PkgA 
can't exist for that platform simply because PkgB - which isn't even 
necessarily required for PkgA to be used - doesn't exist for it.

-J