I'm afraid that we have no plans for releasing titles for any platform other than Windows XP+.
There is a vote up for OS X releases on the new Wishlist, so one thing I can suggest is casting your vote. If anything, it could send us a message that there is a strong demand for these within our userbase.
I would not really expect GOG to officially support any of us salmon types. However, in circumstances where the newer titles have existing versions on other platforms I wouldn't mind if, during negotiations, you guys happened to mention there are a few people that might be interested in the developers having some downloadable solutions to convert the windows version into one of the other said versions. Even if there is a fair amount of unsupported work involved for us.
ID used to have Linux and windows runtimes available for Quake3 and RTCW. You had to had to do some copying of the data files but that was no big deal. If all I had to do was install the game via wine (GOG installers generally work great) , run a runtime installer, and copy some files, then I'm more than happy to do that kind of off road work. I'm sure the community would be fine working it out on the side.
I know its a dev and per game decision and sometimes legal cross-wiring in between versions, but like I said, I wouldn't be mad at you if someone pointed out that some people dig that sort of thing.