Ok, so PX4 and the Pixhawk 4 don't do passthrough of the RC signals if disarmed, however, they do have something similar to the safety feature that you want, I believe.
The way we try to make sure that "manual override" is always possible, especially in the case something of the flight controller should fail, is that the Pixhawk 4 consists of two processors (also called FMU(main) and IO board). The IO part basically, only does mixing and fail-over switching to manual mode if the pilot wants to override using the mode switch, or if the FMU stops sending commands because it has crashed or locked up somehow.
Since the IO code is therefore safety-critical, it is changed seldom and kept fairly simple.
I hope that is helpful (and I'm wondering if that's documented somewhere because it should be).