I am entirely unconvinced that the universal prior really does imply that you should say "substantially less than 1 million".
It seems to me that using the universal prior leads to a probability distribution in which most of the probability goes to hypotheses that take quantum physics seriously, in which case we expect that when you flip a quantum coin a million times we end up with (roughly) 2^1000000 versions of you, almost all of whom see bitstrings with K-complexity about 1 million.
The universal prior should probably also still give some probability to hypotheses in which the universe works non-quantum-mechanically and you get a K-complexity well below 1000000. But not very much.
It seems to me that using the universal prior leads to a probability distribution in which most of the probability goes to hypotheses that take quantum physics seriously
That isn't true, I think. See my reply to Perplexed.
You're about to flip a quantum coin a million times (these days you can even do it on the internet). What's your estimate of the K-complexity of the resulting string, conditional on everything else you've observed in your life so far? The Born rule, combined with the usual counting argument, implies you should say "about 1 million". The universal prior implies you should say "substantially less than 1 million". Which will it be?
EDIT: Wei Dai's comment explains why this post is wrong.