Does it make sense for me to go back and improve an answer to a question that has already been put on hold?
My thought is that it does, because the question may never get deleted, and may even be re-opened at some point.
So long as a question is live on the site, I don't think its status should matter in deciding whether to edit an answer. The same guidelines that would apply to any other edit (substantial, improvement, respect the author's voice, etc) apply.
While you're there, take a look at the question. Could it be worthy of reopening with an edit? If so, can you make that edit? Since your answer-edit is going to bump the question anyway, see if you can help there too.