The original Hobbico lost plane device should work as well with PCM and 2.4 Ghz radios as the FM analog types.

The device looks for a change in the signal to reset it's timer. If the signal stays steady, the device will time out and start beeping.

I must say the the thing is frustrating at first. Before you get your plane started and taxiing out the darn thing is giving the warning and I have to move the ailerons to reset the timer before take off. You get used to it after a few flights. Every time you get close to the transmitter, you wiggle the ailerons.

I suppose on a three channel outfit you would hook it into the rudder channel?