[quote name='supershammy']I've basically looked everywhere, and I can't find the answer to this question: Is it possible to determine what movies Criterion still holds the rights to, and in knowing this, is it possible to determine what movies they have released on DVD that they could release on Blu-ray in the future?[/QUOTE]
That's really a two part question. In theory, if you look at their site, they hold DVD rights to everything that's in print. (I say "in theory" because some of the films that they've lost the rights to are still available in box sets, presumably because the higher price point means they haven't sold out of their stock yet. They should still hold the rights to at least the vast majority of the individual films, though, as when those go out-of-print, they also generally go out-of-stock very quickly.)
However, chances are that for many titles, those are only DVD rights, and to release them on another format like blu-ray, they would have to negotiate a new contract. There's really no way to predict what will happen with those titles; even Criterion themselves don't know until that negotiation happens. There are some cases (like Fear and Loathing in Las Vegas or Chasing Amy) where they have been allowed to keep the DVD rights, but the blu-ray rights have been given elsewhere.
That being said, a lot of the smaller, lesser-known films, the art films, and the foreign films stand their best chance of ever getting a blu-ray release in the US through Criterion, not to mention that Criterion's brand almost certainly exponentially increases their sales, and it's likely that the owners of those films know that. Therefore, I doubt that most of them would be particularly difficult about it if Criterion came knocking and said they wanted to do a blu-ray release. Where the risk is probably more significant is with the bigger-name films and directors.
Criterion themselves have stated that they try their best to keep everything they publish in print, so you can for the most part assume that if it's out-of-print, the rights are no longer there (this is not to say, of course, that they couldn't reacquire them at some point down the road, which has happened on occasion). On the other hand, that statement can also potentially be taken to mean that they intend to release every title in their catalog that they can get the rights to on blu-ray at some point.
Of course, when the blu-ray release will occur is another question entirely. Honestly, how they set their priorities is a complete mystery to me. I mean, don't get me wrong, I think that it's awesome that Robinson Crusoe On Mars is getting a blu-ray release, but how it took priority over titles like Cocteau's Beauty and the Beast, Resnais' Hiroshima Mon Amour, the Olivier Shakespeare titles, etc., I have no idea (I leave the Kurosawa, Bergman, Godard, etc. titles out of that list because I assume they're intentionally spacing those out).