> However, I don't know what a comparably effective alternative would look like
I don't know what the current state of affairs is with regards to radio modem firmware, but I would think that if radio-controlling software should be certified (as following regulation), that should be limited to the firmware, and the modem should only accept firmware updates cryptographically signed by the manufacturer (and possibly the regulator). The firmware should provide an interface that only permits legal use through technical means. IOW, regulation should be limited to the hardware module and the software running inside it. It shouldn't be possible for software residing on any other part of the device to run afoul.
If that's impossible for some reason (which I don't think it should be), then I would argue that other alternatives like focusing on prosecuting violations (like the app and OS you mentioned) or modifying the regulations so they can be contained within the firmware while still meeting goals should come before any idea of locking down whole devices for the regulation of a specific module.
Also,
> You at least have to consider the possibility that someone could create a "High Power Radio" app or OS that would ...
If that's possible then, it's possible now. I mean, you don't even have to consider phones. Bootloaders and OSes in regular computers are open source and unlocked. If that's a problem that can arise from unlocked devices, then it already would have been a problem since long ago.
Additionally, the discussion was not whether there should be unlocked devices, which there already are. The discussion was whether locking should be illegal.