At best, this is a meta-method, rather than a method for interpersonal utility comparisons, since I still don't know which method my uploaded-self would use when choosing between the alternatives.

This is at least useful for creating thought experiments where different ideas have different observable consequences, showing that this isn't meaningless speculation.

At worst, this would only tell us how much utility my uploaded-self gets from (probably) causing a person to exist with a particular mental state and is not actually an interpersonal utility comparison between the two persons.

We have reason to care about the definition of 'utility function' that is used to describe decisions, since those are, by definition, how we decide. Hedonic or preferential functions are only useful insofar as our decision utilities take them into account.