Walter, cool idea and great to have it raised. I agree with some of the replies so far - let it literally be a community decision by having a separate fundraiser targeting logged-in users. if it raises the required amount, put it towards going carbon neutral. if it doesn't, well, some more shrapnel (small change) for WMF. I also agree it would be better on the wikimediafoundation.org site rather than, say, the footer of every WMF project page.
It reminds me a bit of the Linguist List fundraising model. They hold a general fundraiser, and during it they announce "We are going to try and raise funds specifically for project X. the required amount is Y. if we reach the target in Z days, we will fund the project." so donors can vote with their dollars for this or that project. (Some may recall that it's how they raised funds to pay for someone to edit Wikipedia linguistics articles.)
I like it, but I guess it is more appropriate to community-internal fundraising rather than the general-public fundraising we currently do.
cheers, Brianna user:pfctdayelise