For PAL, the baseband video is 6MHz, but the FM bandwidth will be very much larger because FM is a mess of bessel function sidebands (In fact bandwidth is poorly defined for FM).
You could probably do something like use one of the Linear tech microwave mixer ICs to mix down to something like 70MHz, then use a IF filter there to limit the bandwidth to maybe 20MHz, then feed a log amp.
Make the LO come from a programmable PLL and step rapidly between the required frequencies, digitising the log detector output at each step....
I am not actually sure that RSSI is the way to go here however, at 5GHz it is going to suffer severe multipath, possibly something based on the Doppler of the received colour burst would be better, but that would require a real recever per channel.
Regards, Dan.