This is an interesting discussion! I have a theory that I want to toss out regarding the "mysterious" broadcast allegedly of Evil of the Daleks in the 1980s. Does anyone know when fan-made reconstructions began to circulate? Loose Cannon has made one for every lost story. Is it possible the PBS channel aired one of these? Or, perhaps, a telesnap recreation? Remember the soundtracks exist for every missing episode. It's possible there was a special broadcast of one of these. The surviving episode might have also been broadcast though I can't recall if any of the scenes discussed were included.
Thing is, there have been
intensive searches for missing episodes since the late 1970s. The Lost in Time documentary The Missing Years goes into detail about this. Episodes have been found in boot sales, church basements, Nigeria. It is doubtful that any legitimate stone has remained unturned in the US or Canada. That said, it's interesting someone mentioned Northern Canada as a potential source for missing episodes because I do wonder if the archives of, say, CBC Inuvik have been examined.
The sad part is the CBC is really quite disorganized when it comes to this sort of thing. I mean they're fine if you're researching CANADIAN productions - they even have a museum dedicated to CBC history in their main studio in Toronto - but they have displayed such complete indifference to Doctor Who, especially in their recent broadcasts before Space took it over, that I wouldn't be surprised if they did have Marco Polo in a storage room somewhere and it just got chucked because a) it wasn't a CBC show and b) it wasn't Canadian content.
I'm glad we at least have the soundtracks so we can listen to the episodes, and the telesnaps to give an idea of how they looked (something fans of The Avengers would dearly like to have for the almost-completely-lost first season of that show). Earlier today Big Finish released, for free, a 10-minute CGI mini-episode of the Bernice Summerfield spin-off series. I still think that's the way to go for recreating the lost episodes. There's a fantastic fan-made recreation of the Web of Fear trailer on YouTube. I think someone just needs to go ahead and do it - and the software is making it easier all the time - and just offer the finish product to 2 Entertain once it's done. Someone posted a rather cynical comment in an earlier thread on this topic that fans doing this for free are unreliable. Tell that to the Loose Cannon people who for more than a decade, on their own dime, not only put together their own recreations (of professional enough quality that they could be released by 2 Entertain) but they even arranged for cast interviews and behind-the-scenes extras. For example, their 2-VHS recreation of The Daleks Master Plan has an interview with Kevin Stoney, who played the villain and is now deceased and therefore unavailable for any sort of "official" interview should 2 Entertain ever release a version of that story. No one made any money out of the Loose Cannon venture, and still the job got done. I maintain there are plenty of skilled animators out there who would jump at the chance to animate, for example, Tenth Planet 4 and get that story completed, and deliver a product on par with and probably superior to Cosgrove Hall. And the cost to 2 Entertain would probably be less than what it cost them to make a featurette on a "no one gives a s*it about it" story like Time and the Rani!

I certainly don't buy the argument that "The Invasion" didn't sell enough copies. Frankly, if 2 Entertain wasn't selling enough copies of these things, we wouldn't be seeing them continue to release them, with a promise to continue until at least 2013*, now would we.
Alex
*Per a statement made at a Canadian Doctor Who convention last year by Steve Roberts of the Restoration Team. Roberts said, at the time (subject to change, of course), that 2 Entertain intended to continue releasing classic-series Doctor Who until at least the 50th anniversary, by which time all available stories are expected to have been issued. After that, of course, it's anyone's guess as to whether release on Blu-Ray will be possible. Likely not, except for the shot-on-film Spearhead from Space and the TV movie.