> Maintaining your own fork is a ton of work. Even if it's just routinely rebasing on upstream and maintaining your own upgrade infrastructure and doing releases, that's far from trivial.

Well I did it for Mattermost and for some other software as well. Sure, its some work, but it's not "a ton" of work and may not be "trivial" but it is also not "far" from trivial.

Do it like Linux maintainers maintain a ton of patched RPM's, deb's, etc. Just keep a patch in GIT. For every release of Mattermost you do a GIT clone, apply your patch and build it. Most of the time the patch will just apply cleanly. Sometimes you need to make a few adjustments, you make them and put them in GIT. There is no extensive release management or anything. You just build a patched version for every released version.