I'm thinking you have to take off the 4 tiny screws and open up the back of the unit to replace the screw???
That's what I did. The stock screw is too short to grab more than a thread or two.
I disassembled the mount by removing the four tiny screws and got access to the small e-clip that holds the screw in the case when the screw is unseated. I removed the screw and replaced it with an M3 x .5 - 6mm allen head screw. The stock screw is an M3 x .5 - 5mm with the threads removed just below the screw head - where the e-clip fits.
The replacement screw I'm using is threaded all the way, so when I screw it into the case, it does not sit loosely held in by a clip. When I want to lock the release mechanism, I screw the replacement screw all the way in using an allen wrench. It is a bit more awkward to use than the stock setup, and I'm going to ask a machinist friend of mine if he'll cut away the threads like the stock screw and I'll fit an e-clip and have a setup similar to the stock screw. I won't have a torx security head, just an allen head, but the screw will be the correct length.