I’d look into the git-maintenance’s prefetch task. From what I understand, that is more or less what you are looking for. Then just run any old http(s) server and clone them from that https://git-scm.com/docs/git-maintenance

There is also writefreely. It is fairly basic, but says it supports “publish[ing] to multiple blogs from one account”. Haven’t really used it, but it looks kinda cool imo

https://github.com/writefreely/writefreely