Was funny, I figured out a way (before I found out about the gas pressure popping trick) to get the plate off, will end up working well if the mag base plate is stuck on to the point the gas trick won't work. Remove the fill valve, and use a drywall expansion plug. I forget the colour, is the medium or large size, but they are the plastic plugs you use to hang a picture or mount a coat rack to a wall (drill the hole, hammer in the plug, tightening the screw causes the two halves to expand, firmly holding the plug into the wall). Anyways, use one of those, put in fill valve hole, tighten screw a bit to expand the plug, then remove the base. Remove the screw and pull the plug out after.
Incidentally, the big yellow ones work perfectly to remove those tubular locks that cable companies use to block off the cable connection in the cable boxes for TV signals. I only know this because I worked for Rogers for a bit, and if I didn't have the unlocking tool handy, I'd use the expansion plug to grip the inner core to unscrew it. Hence my getting the idea for removing the mag base.
