Re: If Mission to the Unknown were found, would it get it's own releas
I'd say they'd be more likely to add it to any sort of future release of DMP. They've already issued the surviving episodes of DMP in the Lost in Time box set, so they probably wouldn't double-dip people in that way. And it's pushing it enough that they issued Black Orchid and Sontaran Experiment (2-episodes) on their own. DVD-format releases are expected to continue until at least 2013 (according to the Doctor Who Information Network). Maybe by then they'll have found a way to upconvert the old video to high-def, so maybe a reissue on Blu-Ray would include MTTU if it was found by then and the rest of DMP was still missing.
They'd probably sit on it until such a time as they were able to combine it with either a relocated or restored DMP. That or make it available for download through iTunes or something. I've heard whispers that, despite people saying the BBC never wants to animate another episode again after what I guess must have been a debacle over The Invasion (the way people carry on about this point I get visions of landfills in the desert being filled with unsold DVDs of The Invasion), there might still be animated reconstructions done in the future, in particular Tenth Planet Episode 4 which would allow the DVD release of the first Cyberman story and the first regeneration. So anything is possible.
Also, given the quality of the unofficial Loose Cannon reconstructions of these episodes (you'll need to Google on that point) and the commercial release of the off-air soundtracks, someone's bound to just do the animations on their own and either make them available unofficially a la Loose Cannon, or sell them to the BBC. People who say this won't or can't happen need to wake up to the fact it's 2010 - go on YouTube and you'll find home-made animations, including some related to Doctor Who, that are superior to what the BBC did with The Invasion.
The best case scenario, of course, is finding out that in someone's basement is a pristine kinescope of the missing epsiodes, or someone was naughty and walked away with films or tapes that shipped overseas and they get found buried in a crypt or something.
Alex