There's another problem. It's not that easy to feed a separate 0-5V signal into the Transmitter analog voltage pins. It needs at least two cells "live" to run and if you put the variable signal on another pin you have a common ground problem. Unless the radio receiver ground and the battery ground are isolated (which I don't think is true for most BECs) you end up shorting at least one cell for any configuration I could think up. You could do it if the radio Rx has a separate battery supply of course. You can't feed the signal into the current sense input as that is 3.3V digital logic not analog.
Originally Posted by jaccies
It is a nice idea, but not all receivers have a RSSI pin available .
So request to all : Spektrum and JR ( no RSSi) , Futaba ( some have RSSI but inside) the next feature of all your receivers add an outside pin available for RSSI value reading 0-5Volt !!!