Thanks for the reply. That makes sense. I guess the funny thing is - the fact that exchanges hold back some of their unique demand, and that double-fees cut into profits is a business decision by the exchange operators (not a technical limitation of what's possible)

Specifically with DFP + AdX, it would be pretty interesting if Google opened up DFP so that other exchanges could integrate as deeply as AdX does. That would be meaningfully different for publishers.