Source is there, both for MOS and Linux, easily downloaded, and falls under the GPL. As far as I remember the GPL, anyone can use it as long as they upload their own version to the source tree.
The MPlayer source apparently includes a lot of MOS-specific system calls. Porting it to OS4 won't be too easy, and a fully working GCC will be necessary.
Edit: and from what I hear, OS4 will include an ffmpeg.library anyway which will handle movie decoding for its own player.
There's nothing to stop anyone porting it to 68k or WOS AmigaOS now...could be useful.
