I don't know how well the current wield code is designed, though I afraid it is not a very clean desing because of many years of development. I can't estimate what it takes to implement my idea.

But in general, such kind of stuff is implemented using shadows.
You make all changes in a shadow and if every check is passed, then apply changes.
In this case it may be shadow primary/offhand slots or the whole shadow PC (heh, though I don't suggest that!).
In general, it is easier to put a weapon/item in the shadow slot and make all consistency checks at once rather then apply them one by one and make changes in the current wield status along the way. Perhaps some tricks will be needed to make checks for progged items.