Yes, DirectX would have to be ripped out and redone in OpenGL, but so what? 90% of the code would stay intact on the remainder of the source. I still don't see the issue. You could even theoretically make the Linux server versions run natively on Mac already, so why not add the client too? Memory address work may be alot, but bare in mind that there's some time before the Mac versions will even be out, plus once MTA:SA hits 1.1, it's pretty damn stable, so there should be enough free time in between to convert the current source into a Mac-workable version. It would simply be a port of the original. It's not like you're rewriting the whole thing while coding something all-new. I know it's work, but it's still a great idea. Why leave those users out when you can expand? Beat Kye to it. Prove that you're the better multiplayer by working harder than he ever has.
P.S. If you one-up Kye on this, it'll expand your userbase majorly.