As I said, it might have simply been misinformation. I can't seem to find it anymore, which is depressing.
But if no such stat exists, then perhaps Luk->Mag, Con->Luk is my only recourse. If calculating Weight AS loss via Strength is known to cause such problems, I might just follow my own suggestion and remove it entirely. That, fortunately, is easy (if tedious) to achieve in Nightmare. The hardest part about it would be rebalancing things to account for the change, but that was going to be an issue no matter what I did.
Thanks for the help everyone. I might not have gotten exactly what I wanted, but having a course of action allows me to plan ahead and work towards it.