In practice, I would let someone "transfer" wounds to items/allies when needed. But any extra empty wound spots on Items/Allies wouldn't be allowed to be used during the Assassin fight.
I agree with allowing players to transfer wounds on the fly. Otherwise, it's too much of a "gotcha" trap for new players, which most people don't think is fun.
Also, Jason has said several times (including on the wiki) that extra wound boxes on allies are treated just like an extra health box on the hero itself. I'm sure nobody would complain if you moved a wound marker from the 1st box to the 2nd box on your hero's card. Therefore, I see no problem moving it to your 4th box either, even though it might be on a different card.