Des,
I resolved with Dx6i and AR8000 that is able to "use" Preset Failsafe. I can use RTH function with manual switch (and it is simple) but turning off the radio also.
I can't test in flight yet, but through Naza Assistant it works.
In Txcali menu, if I turn off the radio, Naza automatically switch in Failsafe mode and TH at 20% of its value.
I put AR8000 in preset failsafe mode and I binded 6ch Tx with switch in Failsafe mode and TH > 10%.
Max