I have explored how to make a gerrit-substitute and it doesn't seem that hard. Initially, I planned to base on gitloite, but its workflow doesn't suit with gerrit. Basically, you would use a hook to rewrite the push references, and then run a script to register it in the db and UI (I did some simple tests). You just need some glue to make the UI perform git changes (merges) and a fancy display of the changes. Things like command filtering are tricky, but could be obtained from gitolite. The best of making a gerrit-similar solution is that it would be able to show gerrit history, thus fully replacing it (no need to hunt for old changesets in Special:Code/Gerrit/newtool). I expect a functonal gerrit clone could be made in a few weeks. (disclaimer: I don't have such weeks available now, while I may advance it on the future, don't expect me to come with an alternative tomorrow, OTOH if someone wants to give it a go...)