There are plenty of off-the-shelf consensus algorithms/services you could use in the git-hooks that don't have the git-ketch limitation. Zookeeper, etcd, consule, redis, etc. It really just matters your criteria and requirements, they all of trade-offs but I'm sure one fits.

To be frank I think your time is better spent looking into why one of these consensus packages isn't the right call for your needs. Since they could easily plug into the git-hooks & git-shell commands, it could avoid a massive investment of time from your team and you.

A little while ago I spent a night and I was able to get a fully distributed git http server up and running. I used HBase and was able to get it most things working. That allows the only contention to be around refs. Everything else is well distributed and spread across as many machines as you need. Caching is handled on the key value store side, and the http server side. I didn't get to git gc work though.