I suppose you could say my concern is something similar, that the overtravel stop would drift. It drifted slightly in dry fire and kept the hammer from dropping immediately and produced a hang fire.I've also noticed mine drifts slightly, easily tested by setting it to where it's just barely too far out to let the trigger travel all the way back, then work it a few times and eventually it'll drop the hammer. That said, even after about 1500 rounds now I haven't noticed any tangible difference in overtravel, whether the screw is a mm farther in or not doesn't really factor into how it feels. I don't know if it's worth messing with threadlock and any substance that might in a worst case scenario degrade the plastic or interfere with function in something as critical as a trigger when leaving it alone works fine.
That said, in some other hobbies of mine nail polish is frequently used with plastic screws as a threadlocker and will probably serve you fine here. You could also drop a dab of some plastic-safe transparent glue/silicon on the front face of the trigger so it seeps into the screw hole a little and wipe up the excess.
The area around the hole is actually polymer but most of the trigger is polymer-coated steel.As far as not using loctite on plastic, I'm pretty sure the USP trigger is plastic over metal, and so the threads of the overtravel screw hole are probably going through metal.