To be clear, it's not that a receiver couldn't work, but that it shouldn't. A civilian receiver is meant to deactivate under certain conditions lest it be suit

To be clear, it's not that a receiver couldn't work, but that it shouldn't. A civilian receiver is meant to deactivate under certain conditions lest it be suitable for use in things that go very fast and fly very high, e.g. missiles. So as you say, you may be able to change the firmware to disable these restrictions, just don't try and take them out of the US...