I'll try to find time to write up a full blog post response to the challenge, but I have a lot of demands on my time this week. I'm not sure if I'll be able to or not. I think it's a great initiative, though. In the mean time, here are the two charities that I'll suggest if I write one:

1. The Wagner Free Institute of Science, in Philadelphia, already has a Steem account (@phillyhistory). According to their web site, they're currently closed due to the pandemic, so I have to imagine that extra revenue would be very welcome. On one hand, the fact that they have their account and haven't withdrawn their holdings means that they're not hostile to cryptocurrency. But the drawback is that they apparently already gave up on Steem once (I assume due to absence of support), so they might be skeptical of a new initiative.
2. The Brandywine Valley SPCA - who I already mentioned in this post. I think they'd be ideal for the blockchain because daily animal photos are fast and easy to process, and they're social-media friendly. According to their site, they process more than 2,000 animals per year, so that's potential for a lot of unique and original content on the blockchain. The drawback that I just learned is that they apparently operate 4 locations in Pennsylvania and Delaware, so the organization might be larger than the type you're looking for.

Hopefully, I'll find time to do some research and have more to say later in a full post...