Made simple test today by plugging trainer wire from transmitter, which outputs 3 volts PPM, directly to pin 15 of RFD900X.
And connect pin 15 of air unit directly to RCIN pin on PH2.1 controlling 6 servos.
Result: it worked as expected.
However, I felt a slightly more latency compared to normal receiver. It just didn't feel as smooth.
Different radio set up might improve latency. This test is done with default settings except transmission power (100mW)
This setup should be fine for slow vehicles.