There is already implementations of the protocol in the Multiwii Flight controller that run in arduino so it would be basically copy/paste.
I've hacked the current code to add it and it fits perfectly in 328p already full with the Mwc code.
I'll upload it to my site later today so that you can give a look.
I'm currently using BT like you said, on my 9x I made a converter from RS323<>TTL and I plug it in the back of the radio.
And at the same time you still have it showing in the 9X LCD.
On my 9C because it has a FrSky DIY kit inside I actually placed the BT dongle also inside the radio. So it becomes a much cleaner solution.
So far I've hacked one fo the existing frsky apps to add what I needed, log rssi:
But I'm not too happy and I have some other ideas on what I want to do.
PS: I was also looking into putting a buzzer in the TX PCB to signal low RSSI as FrSky does. I've received today a spare ATMEGA from Farnell along with the resistors to swap. So I'll be giving total attention to this in the upcoming weeks.