Another option would be to roll to see if you do re-roll an empty crit. Something along the lines of on a 10+ you do re-roll the crit. To reduce the almost blatant game change that this would cause, any location that has lost 50% or more of its IS must re-roll empty critical slots hit. This will have the effect of larger damging weapons having more chance to hit critical components in lighter units, but allowing those same units to potentially survive for longer against similar weights.
Or you could just make a new IS, let's call it "open frame", that weighs more, +5%, and takes up critical slots in some/most/all locations, every 6th slot on the critical hit table is an "open frame" critical (i.e., the head won't have any, and the centre torso will only have 1, unless you take a compact gyro).
Love it? Hate it? Don't care?