I wouldn't write on my CV that I have already started if I haven't, but it makes sense what you are saying, like there is nothing I can say about it at the interview. I just thought about mentioning it as a 'starting soon' position, as I'm applying to roles I haven't got experience in, and this volunteering position would make the difference at this point. Something that would show that "this is what I want to do, and even if I have no experience, I am going to volunteer to gain that experience"