Dear Laurence,
Thank you for your interesting and inspiring essay where you touch very important issues connected to MUH. In the conclusion you claim: "I have argued that we cannot yet regard the hypothesis as successful. We should not, however, suppose that we have reached the end of the story" ... I fully agree. And in another place: "It is not clear how the concept of mathematical structure is to be defined precisely." That is the point.
MUH can be accepted only if we can find that mathematical structures isomorphic to the reality (empirical domain) and moreover if we can show its predictive power. I have coined Geometrical Universe Hypothesis (related to MUH) that gives the initial conditions using Thurston geometries (the geometrization conjecture) with metrics.
You also see the problem in the flow of time that is "either wholly unreal or at best illusory". The time issue in GUH is quite obvious as the geometrization conjecture was proved by Perelman by showing that the Ricci flow can be continued past the singularities. The Ricci flow is the answer.
"...the result required by the MUH is that all mathematical structures are on a par. If some of them have physical existence, then all of them do." Tegmark is not fully convinced to it. See his another publication: arxiv.org/abs/gr-qc/9704009v2. He proposes "category 1b TOE that may one day turn out to be correct". My view is also close to 1b that only some things that exist mathematically exist physically (our empirical domain), others do not. In this publication Tegmark also touches the self-aware substructures that you are interested in.
My GUH can be valid for any universe, however putting a harness of Thurston geometries on it, we are constrained to the universe we observe.
GUH makes a testable prediction. If you are interested you can find details in my essay.
I would appreciate your comments.
Your essay is one of the best in the contest so deserves very high rating.
Jacek