I distributed source and no one has sued me yet…
There are also source-available quake3 mods, and none of them have had any trouble. Given the history, I suspect even if someone did suddenly decide to pull a SCO, they would have a lot of trouble doing anything more than ordering you to stop distributing it.
Just MHO. I’m not a lawyer, if you want legal advice, go hire one.
Note that the GTKradiant license has similar problems. It actually only allows you to make stuff for quake3, not any of the other games it support :moo: paraphrase quote from ttimo “yeah, we should fix that, but we have more important stuff to do”
edit:
Note that trying to put the modified source under a different license, use it with some other game, or sell it would be more likely to get you in trouble, IMO. Any new code your write is completely your own, of course.