Its definitely fine if you know it gets migrated. As for against the FAQ, I usually tend to hint at an answer when I comment to why I am close voting a post. Some questions are actually interesting enough that I post an answer, then ask, and occasionally when I really KNOW I ought to comment, but it's too long, I've been guilty of posting it as a community wiki - tho I've taken to putting that rep in a pool and bountying it instead.
There's no official policy, and I think its a good thing. You're taking a calculated risk - a closed question can potentially mean lost rep should it be deleted. Trying to answer the question might also help get ideas on how to save it. On occation, halfway through the answer, you realise its an unsalvagable mess, but you learn something new.
I guess as with anything, if its an interesting enough bad question, its worth trying to answer, simply for the learning experience. Someone with a pattern of bad questions will get question banned anyway ;p