private repo they commit to and build from

This isn't possible with Ruby and Mastodon. The only way to distribute the patch is to reveal the changes to the source. FWIW, compiling the fix is still just an obfuscation method, one can still just diff the binaries and see what changed (see: reverse-engineering Windows vulnerabilities in updates).

At best, you can release it with a bunch of unrelated and obfuscating changes, but putting work into doing that is further delaying simply getting the fix released.