I just don't see how. It seems like a fairly obvious point that the two systems would be indistinguishable from the outside.

In that case, the pragmatists would say that the distinction isn't meaningful, that free will is a concept with no meaning, and end the debate there.

I don't think it's plausible to maintain that the question of whether or not we have free will is "meaningless". Any criterion of meaningfulness that derives that conclusion is probably faulty.