I wasn't analogising, I'm just saying the music industry must surely be a rich source of case law regarding joint ownership works. The case we're looking for is where a rights-holder goes genuinely AWOL, and the other rights-holders agree to something in their absence, with the AWOL rights holder popping up again, well after the fact, and objecting.
I don't really know how to search for cases, just curious if anyone does.
Basically, I strongly suspect the "we can't contact everyone" thing might not be much of an obstacle. If you make all reasonable efforts to contact known rights-holders and publicise a proposed change, over a longish period of time, then I suspect that'd be enough. Like coriordan writes in his piece, if you can show a near-unanimous acceptance of a change amongst known rights-holders, and no objections, then I doubt a judge is going to think it reasonable to prejudice all their interests in the work for the sake of a few AWOL owners. But hey..
TBH, I suspect the /contactable/ rights-holders would be much more of an obstacle ;).