The competition aspect would add something missing from from charity run/walk thingies. I think that your biggest problem would lie in getting the word out, but once you did, seems like it should be able to snowball from there. If you could advertise with venues that have major pull with your target audience, then you would have it made.
Child's Play is a great example of knowing your target audience and appealing to their sensibilities. I think that a problem may lie in the pledge donation facet of it, however. Maybe corporate sponsorships by video game companies could help with this?