Interesting approach. The way I would put it, the number you want to maximize is expectation of U(X) where U is a utility function and X is a random universe taken from a Solomonoff ensemble. The way you put it, the number is the same but you don't interpret the sum over universes as expectation value, you just take it to be part of your utility function.

What I feel is missing in your approach is that U by its nature is arbitrary / complex, whereas the Solomonoff prior is simple and for some reason has to be there. I.e. something would go wrong on the philosophical level (not just the pragmatic / normality level), if you throw Solomonoff out of the window. But, at the moment I can't say what that would be - it's just a hunch.