I think we missed something in the whole P2 ride, happy that HUBexec replaces LMM. But we did thought a little bit to short.
There should have been a interrupt source if HUB acces is out of HUB size. Especially since a Family of P2 was planned with different HUB sizes.
Like the Debug interrupt it should run user code, handle the HUB access out of range and be able to resume, if no handler there, do as before and just wrap the HUB ram content.