I assume you mean, setup with a bitcoin-accepting third-party who can donate the equivalent amount of cash, in order to expand the list to accept any donation targets, not just those accepting BTC...?
It's an option, but I like your comment about rewarding those who already accept BTC. Also, we'd probably pay a fee to whatever third-party, which makes it even less attractive.
Yes, that's what I had thought; or, equivalently, someone selling the BTC and donating the funds. If we could find people willing to do the exchange without charging (I'm sure we could find a few, maybe not 20), then the only fees would be external: BTC/fiat conversion (if any), fiat-to-fiat processing fees (probably quite low), etc. Still, better to reward those who accept BTC or begin accepting it for this - a 4 BTC (~$2,480) reward for setting up a free account should be enough for many charities to bite.
Humble Bundle is a good one, but I say that only because I'm personally a fan of Humble Bundle. They were early entrants into the Bitcoin scene, and ATI has donated to them before. Not sure if they should be part of the promotion though -- we'd like to support charities that have wide appeal and Humble Bundle only connects with a portion of the Bitcoin space (i.e. only the bitcoiners who play games). I'm still happy to entertain them depending on how many options we have and how much in donations we ultimately decide to match.
(I was just using it as a source/list of some eligible charities, but now that you mention it...) We could give to some of these charities through Humble Bundle. It would depend on what charities are supported by the bundles available at the time (not counting their Store, which has a fixed donation percentage).
E.g. the current list is: Red Cross, Action Against Hunger, First Book, IndieCade, SFWA (Science Fiction and Fantasy Writers of America) emergency medical fund. Giving one bitcoin would almost certainly put us on the top contributors list of whatever bundle is bought, and so can also serve as a bit of advertisement for Armory/Bitcoin (and the matcher(s)?).
We could also give to Humble Bundle (which is what your post suggests, as I'm interpreting it), but I agree with your points on that, and I see them as a business, not a charity - so no.