"Most receivers have a "signal strenght" value somewhere (for automatic gain correction purposes)
feed that value to an a/d converter and base uour design on that."

I don't think that will work - I think that the "nice" variation of RSSI with distance will be totally swamped by the peaks & nulls you get due to multipath effects.

I speak from experience with both UHF & VHF radio mics: the received signal strength doesn't just drop off predictably the further away the transmitter gets - you find that that the transmitter only has to move a very small distance, and the signal will drop out completely; another small movement, and it's right back again!
This is why any half-decent radio mic system uses a Diversity receiver.