Found it: it was actually in Character Law (red band version, p. 18). It is just a small chart listing what to do for armors that are heavier than normal, in a chart with the title, 'Bonuses Due To Heavier Design':
You give a +5 bonus for items up to 120% weight, but these have a +5 penalty to maneuvers and cost 2x normal.
Similarly, you give a +10 bonus, a +10 penalty to maneuvers, and a 5x cost multiplier to items up to 140% of normal weight.
You give a +15 bonus, a +10 penalty to maneuvers, and a 10x cost modifier to items that are up to 160% of weight.
That's all for that chart, but you can see the basic principle clearly. I thought it was in one of the companions, but it is actually in the core RM2 rules.