I wouldn't recommend feeding any rats that have Ivermectin in their system *now*. I say you can probably feed them off later, because Ivermectin will be gone by then. I'm not saying that it will be all gone in 3 weeks. It might. But if it is still there, it won't be as strong/concentrated then compared to now. Not as risky...
But Jerry (snakesRkewl) said he's fed off treated rats fine without any negative effect to the snakes. So that is really your call in the end. It's your snakes and rats.