I'm not in the US so can't say if Social Security would be contacted or you would be asked at some point to report your volunteering hours but if it is only a few hours a week I doubt it could have any impact on your evaluation. Someone who has volunteered on SS disability would know the answer.

Volunteering can have many positive impacts and you would also have a unique perspective since you had been a patient there too. Eight years is a long time and I would hope they would not hold your previous status against you in applying.