I have an RC that applies only to a certain series of books in HB.  Several of these are on my WL.

I would rather not have the RC on all the time, so that it doesn't annoy anyone when I'm ordering books that are not in that set.  If I have the RC off and get offered one of these on my WL, can I then go in, turn it on, and accept the book?  Will it still show the RC on the request?  or does it need to be on before I even get the offer?

As long as you do not have the WL book set to auto-request, it will work. You will get the offer, turn on your RCs, then accept the offer. This is what a lot of people do to avoid confusion on their other WL books.

I will do that then.  I want to avoid confusion on the other requests but wasn't sure if it would work that way.  Thanks!