In short, it would be useful to allow posting answers for questions that are within the "on hold" period.
If need be, and probably wise, the ability to do so could be linked with an appropriate privilege.
The reason I bring this up is because I was just about to post an answer to a question when it was closed - this one actually, which is already well on it's way to being re-opened anyway. It was quite annoying as in this case I have wasted my time posting an answer, to what in my opinion (and yes I know not everyone will share my opinion) is a valid question.
I could wait until the question is re-opened but that causes the following problems:
It may never be re-opened, and some closed questions deserve to have an answer
It annoys me when I waste my time typing an answer and can't post it because it has been closed within minutes, by users who probably don't understand the question to start with
In the event the question does get re-opened then it might be too late by then. For example, if the question I linked isn't opened in the next 30 minutes I will be gone for the weekend, my notepad answer lost, and I will likely forget about the question when I am next on SO. So the OP doesn't even get the answer they deserve
Now I know a lot of question are closed for a good reason, and we probably don't want answers posted on those, but the reality is that users with enough rep to post answers to "on hold" questions will know when it is appropriate to post based on experience.
Also, you only get the 5 day "on hold" window until the question is fully closed anyway - at which point I would agree enough time has passed to say definitely no more answers, no matter how much rep you have.
EDIT: The question I linked has just been re-opened. But naturally this isn't going to be the only instance this applies