FYI, the Wren team reached back to Gaspar on the quesiton of the WREN FADEC ECU with the newer receivers. As usual, they went the extra mile to get a good answer to a customer question. With their agreement, I'm posting what came back from Wren via Gaspar:
Here it is:

*****************************
Hello David,

We've been talking to Gaspar, and he says the ECU is fine: see below.
He's obvioulsy following the discussion on RCUniverse.

Regards
Sara


Don't worry, there isn't any potential problem with the ecus, they
work with signals as low as 1V. There are some servos, sequencers and
powerboxes that have a very "simple" RX input that don't work with
signals under 2,5V, thus giving issues, and this is what is being
discussed now on RCU.

Gaspar

****************************