Ok, not to diss this project or anything, but relay-proxy from earlier this year does the same thing. Multiplextr does as well, but using a wrapper protocol so clients can make non-static relay selections.

Proxies like this hurt the ability for clients to intelligently route. They also have significant privacy drawbacks, especially if proxying AUTH (they can exfiltrate protected data).

I'd love to see more work in this area, especially since proxies have the ability to improve routing without clients having to do anything by inspecting filters coming through. I recently wrote a bit about this, see below: