Eliezer has expressed the idea that using a Solomonoff-type prior over all programs doesn't mean you believe the universe to be computable - it just means you're trying to outperform all other (ETA: strike the word "other") computable agents. This position took me a lot of time to parse, but now I consider it completely correct. Unfortunately the reason it's correct is not easy to express in words, it's just some sort of free-floating math idea in my head.

Not sure how exactly this position meshes with UDT, though.