[Sethomas]

As DK expressed, it shouldn't ever be an issue. If it is an issue, as my life can attest that non-issues are often interpreted as being issues, then at the very least you can make your case. None of us here can judge how sound your reasoning is about being able to sporadically volunteer but not hold a job, but since you have your disability checks you can say that the government made that judgment for us.

In the odd chance that it's something they watch, you might want to declare your volunteer intentions before you do it. At the very worst you'd end up volunteering clerical shit contrary to your desire to volunteer hoisting massive boulders over your head and throwing them fantastic distances for the public good.