I was also just reading an article about how 37 Signals prevents its employees from working more than 4 days per week during the summer (here in SF that should be during May/June and Sept/Oct because who wants to spend an extra day not working in the foggy cold? I digress).
How many people already have a volunteer opportunity that they can plug into? Seems not so easy to do, but perhaps that's the point.
I love this idea of encouraging people to both take a little time for themselves and for others! It may seem like a sacrifice to lose 2 work days a month, but you may find that those 2 days allow you to be more focused and productive during the rest of the month.