Since the game doesn't model the manpower expansion that went into industry during the period, if you want to make IC cost manpower, then you have to re-work the entire manpower availability data to account for worker manpower and worker manpower additions. Thus, MP streams would be *increased* in the majors, since all of them expanded their workforce and industry during the war.
Isn't that what industrial and economic laws model? Also, don't certain decisions with IC buffs model various efficiencies states found in using their industrial manpower more effectively? Also, don't the industrial techs model increased efficiency of both workers and industrial processes? Hell, you wanna turn the Soviets into the #2 economy in the world, you build 100 base IC, but you also research the IC techs using construction practicals. A Soviet Union with up to date IC techs is pretty damn scary. The only reason they can't catch the USA is because the USA has such a large head start on the Soviets.
I'm talking about strictly building additional IC. If I build a new factory, someone has to work there. If I expand an old factory, more people have to work there. If my existing workers get more efficient, that's an IC tech (increased IC percent added to base IC). If my industrial processes get better, that's a different IC tech (reduction in IC/days). If I mobilize the economy for war, that's economic laws. If I gear my industry from consumer goods to weapons, that's a conversion from CGO to Heavy Industry or Mixed Industry.