This seems like a halfway to federation approach which I have no issue with. My issue is that access to the data on relay should be inaccessible by relay operators unless specifically granted by user for specific time and task. This probably wont happen though since it would imply moving the filtering / trusting capabilities to the edge by necessity and i dont see relay operators wanting to give up access to peoples data. 

Anything less than this while relying on relays leaves the door open for abuse. doesnt matter if the current devs and relay runners are saints. We dont know who comes next. its a common problem. once sysadmins, dbas, SLA holders get their hands on data they cant help themselves. eventually they start poking around. they conflate "their server" with "their data"

My true concern is that out of expedience we are setting the groundwork for future abuse and social engineering.

Good luck as you proceed.
I will watch with interest.