I love this concept. It combines the concept of volunteer work while keeping mobility issues into consideration. I am a strong believer that a sense of purpose is the strongest factor against ageing and depression.
A suggestion would be to make this a part of the schedule/list of programmes at senior centres/retirement homes. Annie's ideas of nursing home based volunteering opportunities as well as connected communities centres for the same is bang on. Maybe established volunteer networks like VSO, ActionAid and so many more out there can introduce a senior volunteer component (if they don't have one already) in partnership with such centres? That could minimise exploitation and enable distance (?) volunteering.