One nice thing about the hashrate escrow mechanism is that it is indifferent to what's on the other side of the hashrate escrow. Could be a drivechain, could be some other flavor of sidechain, could be a chaumian mint of some kind... imo this flexibility makes up for the otherwise "single purpose" nature of the BIP. If hashrate escrows could be implemented with a multipurpose op code that would be an interesting result, assuming the other capabilities of the op code were themselves desirable. I remember zmn posting about being able to implement drivechains via recursive covenants, though in that context zmn was using this as an argument against recursive covenant op codes rather than for them.