Noodles: that sounds like a wonderful opportunity! And what you have put down sounds perfectly adequate. I train and supervise volunteers at our community garden, and have done so in assorted national parks, conservation and restoration projects, bushfire recovery projects and wildflower gardens, and I would be quite happy with what you have said as reasons for volunteering. I agree with Sc0ut; I reckon they will be happy to get you as a volunteer. Good luck!
Sc0ut: I think it is quite reasonable to ask your roommate to give you warning of the presence of the dog, even if only on the level of a quick tap on the door and a mention that X and his dog have just arrived. This would be good both for you and for the dog, since it's quite possible that you might otherwise leave something lying about which may be dangerous to the dog (chocolate, tobacco, onions and tubes of oil paint spring to mind). And it is always unwise to startle a large animal, even a well-trained and friendly one (says the person who was once knocked out by a large friendly dog leaping up and embracing her).