Since you're talking about involving charities, why not make the contest about giving to the TF2 community, rather than just involving our community.
I still think a 72 hour mapping contest isn't going to produce terribly great maps. A flood of 100 okay maps isn't going to benefit the community, and similary when we start talking about collaborating within 72 hours, I think the quality is only going to suffer.
If you focus on giving to the larger TF2 community, then maybe making full maps isn't what we want in the first place. What if it was 72 hours for TF2Maps members to work on a content pack to aid mappers. It could be in the same vein as the swamp/construction packs, but wider scope. People could work on props, prefabs, scripts, 3rd party tools, map templates, vmf galleries, etc. 72 hours would be enough time to produce really useful stuff. It would also be easier to involve Valve in this, because at the end Valve could (with some amount of quality control) release it like they did the Swamp pack. The charity part could be that the money Valve would pay to purchase it goes to a charity instead, or something. There are multiple ways you could monetize it.