Well, there are two problems I see here really. The first being that we don't actually know Ambrosia's legal rights in terms of the source for Coldstone. Do they own it outright? Do they have merely the right to publish and perhaps update it? Do they have almost nothing at all?
Only person that knows those answers would be Andrew, and he's unlikely to spill confidential business agreements in public.
The other problem has to do with contacting Beenox about it - and it's a multilayered problem. The first layer being that we don't know, as above, the relationship between Beenox & ASW in terms of specific ownership... and the second problem, namely that Beenox doesn't really exist anymore; they are a fully owned subsidiary of Activision, and aren't independant at all. As such, I don't know if they have any ability to release their old code, even if they retain any rights to it - it's possible, even probable, that Activision bought up their back catalogue as well, which means you'd need Activision's permission, possibly Ambrosia's permission, maybe even some other people's.
It's a nice idea, really. But we'd still need other things than just a copy of the source code... there're a lot of great OSS projects that just fold up and die because there's either not enough interest, or not enough skilled programmers to go around. Given Coldstone's current woes and limited usability (compiles on what, PPC X.2 only?) it might not even take off as OSS.