Would it be against the rules to add "caution rotor wash - recommend 2 minute hold"? Might help, especially if it's a low hour pilot (like me)!
It might be legally possible - would be a question for SRG. Consider though that if we have say five aircraft in the cct and we warn someone and recommend a two min delay - we would then have cct traffic reaching final with the departure sat on the runway, as we can't tell aircraft to orbit/ extend downwind etc to fit in with what's happening on the runway. This would increase the number of go-arounds.
It's not really too much of an issue anyway as most of our helicopters don't join to the active.