If someone wants to work off my private stuff I would tell them "sure, but be careful cause I'm push -f'ing" (after all, it's usually pretty easy to fix) and give them a heads-up when I do. If that weren't acceptable, I'd add a tag like "stable" to my branch, tell them to use it only up to that tag, and move the tag forward as the work progresses. If that weren't acceptable I'd make a branch instead of a tag and tell them to use that.