Unfortunately, 2.4GHz is very much like PCM insomuch as there is no inter-brand compatibility.
If you have a Futaba FASST radio then you're stuck buying Futaba FASST receivers, nothing else will work.
I'm finding that the Futaba 2.4 FASST T7C transmitter seems to work correctly with Hobby King's FrSky TFR6 receivers. I was skeptical of HK's advertising the TFR6 as "FASST Compatible" but decided to give it a try. It does lock up to the Tx with no problem and seems to function like the original Futaba R617FS receiver.
Even though I got a good price on the original Futaba Tx/Rx package (from a US dealer) I later discovered how ridiculously expensive separate receivers are and was kicking myself for getting caught by a corporate "trick". I do rather like the Futaba transmitter and will be very pleased if these more reasonably-priced receivers will do the job since the FrSky TFR6's are a quarter the price of the Futaba R617FS's.
I don't have a feel yet for how reliable the FrSky equipment is. Has anyone else tried this combination?