So I have responses, but they're moot -- I found the Cartesian boundary.

Hypotheses in which AIXI's actions already have no effect on the environment are useless for action guidance; all actions have the same utility.

Fortunately they get falsified and zeroed out right away.

I'm skeptical that this sort of thing could ever end up as a leading hypothesis.

The leading hypothesis has to not get falsified; what you've described is the bare minimum required for a Solomonoff inductor to account for an AIXI agent in the environment.