I worked at a place that was allergic to contributing patches upstream. We maintained a lot of internal forks for things and had no problem collaborating.

You don't need to push the rebased branch to the same branch on your remote, if that's an issue (although I don't see how it is).

Maybe this is a case of "Dropbox is just rsync", but I feel like just learning git and using it is easier than learning a new tool.