This seems unduly complex.

I really like the mechanic at first glance. It may confuse people about blocker damage ordering though.

You can only play it before assigning damage.

Er, that's technically true because damage is assigned immediately before it's dealt now. But if I block two creatures on a Hundred-Handed One, for example, at the moment of declaring blockers I need to declare an order for which of your attackers I'm going to damage first (just like you need to declare which of my gang-blockers you're going to damage first). Then if you drop in an assist creature blocked by my Hundred-Handed One as well, where does it go in the damage order? As it happens the rules do state the answer, but it's the kind of thing I suspect many players won't know, even expert players.

509.6. If a spell or ability causes a creature on the battlefield to block an attacking creature, ... Then the defending player announces the attacking creature's placement in the blocking creature's damage assignment order. The relative order among the remaining attacking creatures is unchanged. This is done as part of the blocking effect.

In the worse case, we can default to last in the order.