Covering a few points related to Haze's newest posts:
1) If stuff gets mislicensed, it's quite obvious and easy to fix. Kale did not need to engage in Mortal Kombat to correct drivers/jaguar.c, just a text editor. If he's concerned about this, Haze needs to start license-tagging his files like everyone else is doing.
2) Since MAME will continue to be distributed under the MAME license, all files in the project are implicitly dual-licensed with the MAME license (we should probably note that in the documentation). Therefore, there is an easy path for a deco16-like combination: you put the combined file under the set of licenses that's common to all of the originals. A MAME+BSD file combined with a MAME+LGPL file would end up MAME-only, for instance, and that's completely legal. Now, of course, you'd want to not be a dick and talk to the original authors first, but legally you wouldn't have to.
At the same time, I expect any conflicts doing this to be rare; it looks like a majority of devs are fine with any license as long as it's not closed source so MAME+BSD will be a popular option.