tankhunter, i think your proposed solution is spot on. removing the mag catch lever and requiring a small tool (like a pin punch) to release the mag sounds like it hits the requirements perfectly. In fact, I even wonder if the second bit about welding the mag catch pin is even required. Per the law the magazine just cannot be "removed readily". By the same definition removal by disassembly of the action does not meet the requirements of "removed readily".

In any event I'm having the Bren sent to my local 07 FFL dealer. He will have to ensure compliance before transferring it to me. I'll let you guys know how it goes.