SI belongs to a class of priors that could be described as "almost computable" in a certain technical sense. The term is lower-semicomputable semimeasure. An interesting thing about SI is that it's also optimal (up to a constant) within its own class, not just better than all puny computable priors. The uncomputable prior you mention does not belong to that class, in some sense it's "more uncomputable" than SI.