The inner, hi-pressure tube acts like a 360* rim lock, in addition to the rim lock of the system, holding the tire bead to the rim.  Hi-pressure air to seat the bead may not be necessary as long as the inner, hi-pressure tube is inflated first.  The hi-pressure tube should prevent the bead from coming unseated when running really low tire pressures.

Plugging a hole in the tire still mounted on the rim is the pretty much the same as plugging a hole in a car tire still mounted on the rim.  The only difference is when you insert the plug into the tire, you should insert it at angle to avoid puncturing the inner tube.  As long as the hi-pressure tube is still inflated, the bead should stay seated.