There is a lot of speculation about this but nobody seems to really have hard insight. As long as the hardware supports it it should in theory be possible to unlock it but like with those 48G 4090's that can lead to weird side effects. For instance, you would never know that these cards are modded until you start using p2p and then suddenly it all falls apart. The reason is that for p2p to work they map the memory space of one card into the memory space of another. But if the BAR1 is < than the total VRAM that trick will work only until you run past the TOP of that block and then you'll have a crash.

I suspect that modding a 5090 in such a way that you can sell it is a lot harder than doing the same with a 4090.