I would actually have objections to that.. Reason being is this: the legality of emulators (for this sort of purpose) is teetering on the borderline, especially with the DMCA (which, in my opinion is the CRAPPIEST PIECE OF LEGISLATION EVER PASSED.. EVER!) which allows pretty much ANYTHING to be ruled as a tool for circumventing copy-protection and thus be deemed illegal.
That said, adding a d3d renderer would obviously be PURELY for the purpose of making the games playable, and not for trying to preserve the actual hardware in software emulation. That would sort of destroy that whole argument, which I don't think would be good, considering how sue happy everyone is lately, especially corporations, who are struggling from the lackluster economy.
Just my opinion tho.. Then again, I'm not a mame dev, so I really have no say in it