Also (chuckles to self) the idea that MODO would program the new mulligan + Serum Powder interaction correctly out of the gate is low probability as well.
I don't think so, they probably have a mulligan counter or something and that doesn't move up if you use powder, which wouldn't change with the new rule.
BTW, if that happens MTG will be very different. I don't know yet how every piece will move, at least G2/G3, decks that have very key pieces (dredge, survival, to some extend shops, etc) will have more chance to find their pieces, as well as their opponents will have more chances of finding hateful keepable hands. The only thing I'm sure about is that it would change things a lot.