Shannon, it seems that REX engine won't be 64 bits native, so you couldn't write a 64bits Poise version in a future if you implement it, I think. I assume that there will be more ppl interested in 64bits than rex files.
I read that on IL forums when someone were asking for a native 64 bits FL Studio, I think, I don't remember where I read that. You know, those are one of the things you read one time and you think "I'm lucky I didn't buy that!" then you forgot.
You could write to Propellerheads to ask them personally.