In a dovetailed ensemble universe, it doesn't even really make sense to talk about any 'basement' reality, since the UTM computing the ensemble eventually computes itself, ad infinitum.

I don't get this. Consider the following ordering of programs; T' < T iff T can simulate T'. More precisely:

T' < T iff for each x' there exists an x such that T'(x') = T(x)

It's not immediately clear to me that this ordering shouldn't have any least elements. If it did, such elements could be thought of as basements. I don't have any idea about whether or not we could be part of such a basement computation.

I still think your distinction between products of cosmological-type optimization processes and agent-type optimization processes is important though.