I too decided late, after my second year of college. I think that a year is plenty of time to get in clinical/volunteer experience. It would all depend on where you get the experience, and how much of it you get. If you volunteer 2x a week for a year and vary the places where you go, that'd be fine I think! You could also do a variety of day long or week long shadowing experiences with physicians of different specialties to show the committees you've gotten some good exposure to the medical field. I'd also strongly recommend that you do some volunteer time in a hospital, just to see a variety of settings.
If after all that, you're still worried, you could get a LOR from a volunteer coordinator. I was considering getting a LOR from the charge nurse at the Shock Trauma center where I volunteered, because she trained me and I worked with her for every shift. It would be good to get a LOR from a physician you've shadowed or volunteered with on a consistent basis. Finally, if you are considering applying to DO schools (as a nontrad/late applicant, I'd strongly recommend casting your net widely), keep in mind that many require LORs from a DO physician. I've heard on SDN that there are exceptions made sometimes and LORs from MDs are allowed, but personally I wouldn't take that chance. Anyway, I think if you make a good, year-long plan for volunteering, doing well on the MCAT, find some sort of leadership EC, and apply to a variety of schools, you can be competitive.