> the problem is one unscrupulous party is unscrupulous to different parties and the different parties at different times are unaware of it

Sure, but eventually you get called out on it in a public forum, like this one, and people stop giving you goodies going forward. I would consider it acceptable practice to, when considering dealing with OpenBSD (or people who are close to them), (a) withhold vulnerabilities until after the embargo date or (b) refuse to give any information unless they sign a binding non-disclosure agreement committing them to the deadline under pain of penalty. (The latter is an option because it appears, in this case, they broke the spirit if not letter of the agreement. The solution to that problem is legalese.)