-While we will gladly accept traffic from almost anywhere at the moment, we are restricted to English-speaking audiences.
-That isn't an issue at all: we don't require a continuos 'spot' per se, a segment of your monthly traffic would be perfect.
-While the size itself is flexible to an extent, we do try to ensure our ads are subtle and non-intrusive, and the 250x100 rectangle is a perfect medium for that. Furthermore, the Leaderboard and Tower formats really aren't suited to the static (no distracting animations) design we utilize.
You should really re-think your stance on the size. As a website publisher, anyone wanting to run a non-standard size is going to have to A: pay me for an exclusive spot, because I can't run any other ads in that slot and B: pay for my time to write custom code for your ad. Doesn't happen often.
That's all assuming that's a size I can even fit into my design without making the site look bad. The 250px width is awkward. For us, our right rail is sized around a 300px wide big box ad, 250 is enough off of that that it's going to ruin the solid right-side margin we've designed.
Thanks for the details Tyler! The size is of course flexible, by suggesting 250x100 I was just trying to give a general idea of the small, non-intrusive style we're requesting. I've updated the post to make this clearer.
I don't know what traffic, if any, I'd be able to donate, but if it was more than 0 then most likely leaderboard and tower impressions are where it would be - MPUs are usually completely sold out.
As to non-intrusive, as a general policy it's something I agree on, and we look for in our paid clients (possibly not to the same extent as you, I don't know), but ultimately our sites still fit traditional banner advert sizes.
Do the charities not mind about nationality of their audiences? Even if they don't, it often makes a big difference to publishers. For example in my case, US impressions are very different to UK impressions, which are very different to Swedish impressions... etc.
edit: I also completely second TylerE's comment, he's absolutely right.
Yeah, it's great. It is already being done, but that doesn't mean there's not room for more competition. You could probably outsource the printing and I still think the coffee house crowd is the right demographic for charitable advertisements.
Given the fantastic response levels to this I'd hope to see more information about the group, the charities, and more extensive financial disclosure. It's all too easy to claim great utilization rates and great recipients. That said, it seems a very worthy cause and you seem to have picked a good place to promote it.