I've never seen one, but I do have a client that alows employees to participate in volunteerism. For example, there is a program up here in April called "Rebuliding Together". It is held on a Saturday, and I know of a company that allows employees to participate. If they participate, they get to take a day off later in the year. Another company sponsors a blood drive once a year. They allow employees to act as volunteers, and the employees are paid as though they were doing their normal jobs.
I would imagine that any policy would require advanced approval of the organization and the limits of the volunteer work (time,travel, expenses... etc.)