I've described this. Downstreams of the feature branch rebase from their previous feature branch merge base (a tag for which is left behind to make it easy to find it) --onto the new feature branch head.

E.g., here's what the feature branch goes through:

feature$ git tag feature_05

<time passes; some downstreams push to feature branch/remote>

feature$ git fetch origin feature$ git rebase origin/master feature$ git tag feature_06

And here's what a downstream of the feature branch goes through:

downstream$ git fetch feature_remote

<time passes; this downstream does not push in time for the feature branch's rebase>

downstream$ git rebase --onto feature_remote/feature_06 feature_remote/feature_05

Easy peasy. The key is to make it easy to find the previous merge base and then use git rebase --onto to rebase from the old merge base to the new merge base.

Everybody rebases all the time. Everybody except the true master -- that one [almost] never rebases (at Sun it would happen once in a blue moon).