One problem with this: what if the figure is already Borrowing?
Then you have a choice, either there is no additional affect to the figure, or they are now Borrowing an additional action.
I would go with the second, so the figure has more than one Borrowed action. Each Borrow requires one Action to clear, so it would just take longer. We use that for creatures with a freeze or stun ability as well, and it works very nicely.
The only issue I see with the one shot idea is that you then need to mark the figure to note that the weapon has been used, and mark the figures to show that their shield is no longer in use.
If playing a game where the figures cost is important (used as Victory Points perhaps), what happens to the cost if the shield is factored out, or the Pilum has been used?
I'm just asking questions, bringing up thoughts. Anything will work, and will work just fine, which is why I dearly love these rules.