It won't. It requires a hard fork and Bitcoin will not hard fork for DarkSend, nor jeopardize regulation issues.
It doesn't require a hardfork if the coinjoin organizers aren't paid by block reward, but instead by those they are coinjoining for. Or better yet, it would be decentralized and no one would be paid at all.
Re: regulation, the Bitcoin foundation is claiming transparency etc etc. If they go the "shady" route, they can't claim the same.
Re: hardfork, how do you provide a dis-incentive for having nodes that map coinjoin transactions in your scenario?
TL;DR PoR is an oxymoron because you can't construct a proof that you are offering a resource.
If you delay payments for a couple of blocks, can't everyone see who did actually offer a service and pay them? There is no hurry to pay in the same block.