Well, the thing is that you'd have to model
every specific type of bomb to do that. If you gave the allied 1000lbser an explosive charge of, say 380kg some guy might pop up and say that because he flies Spits, he should have a british mark XY bomb that actually had 392kg of explosives. LW haters (make that "dislikers"

) might argue that the german 500kg type I was indeed used on this-and-that plane and therefore should have only 391kg payload as opposed to the 410 modelled.
I'm quite in favour of calculating lbs and kg weights exactly, by a factor of 0.45359. Anything further than that seems low priority to me ATM, there's more important stuff to be done.