It always annoys me when RAF controllers phone up and say 'can you restrict your (squawk) not above XXXX ft?'; I reply 'it's on a FIS, so I can't restrict, I will REQUEST, but it's really up to the pilot'.
For my own RAS/RIS traffic against identified FIS I would say 'advise if you go above XXXX Ft; I'll be descending inbound traffic to 1000ft above you'. It's not an instruction but a reasonable (to my mind) request with safety in mind. If the FIS then decides to go above, I'll always have the bolthole of avoiding horizontally.