Off the bat, I don't know a thing about your setting, however:
I'm not sure if i'd straight substitute heat criticals, just because they tend towards the 'target on fire', flamethrower-y end of things. I'd probably use mana criticals and ignore or co-opt the powerpoint critical effects. In fact for most of the energy criticals I'd use mana as a subtitute, supposing that you couldn't get your hands on the plasma critical table.
The powerarmor and force field suggestions sound good. Depending on the type of field, a intertial dampening field sounds like a classic case of critical reduction (sure it still goes in your thigh, but less deep). Whereas i'd imagine deflection type shields would be more towards the DB bonuses.
Powerarmor would also likely have far fewer MM penalties (total and minimum) than your typical suit of AT 19 or 20. Heck even just a suit of At 20 that I don't need a lifetime of skill to be able to run around in would be nice, to say nothing of STR bonuses or critical reduction. Some of the STR bonus would probably go to just that, being able to manuver in the bulky suit without it overencumbering.