Hi! Great question.
There's always the possibility SSA can check into your activities/activity level, of course.
I'm suggesting you have either your pdoc or primary care
(or both) write the volunteering into your treatment plan, listing it's potentially positive effects. This would likely cover you should your claim be investigated.
I do this for any regular activity, including things like going to the gym and/or using an inside track for walking, walking at the mall, etc. It's all a part of a documented treatment plan, "prescribed" by a physician.
Just a thought.
Good luck with this venture!

WC