Perhaps with this feature, 6166 would be unnecessary.

Interresting.

I have an implementation of this from December (in http://github.com/bap2000/plugin-promoted-builds.git), but it has not yet been merged.

It adds a new action to the list available in promotions which will "Keep Build Forever".
If you create a promotion with the "Keep Build Forever" action, then it will get the lock whenever the build achieves that level and therefore not get cleaned by the housekeeper.

... actually just looked at the code for it and it appears I'm doing something silly!
It does a check to see if the original build is good enough for keeping, as well as the current promotion process.
Who am I to decide whether the original build is good enough?

Let me re-fork from the current plugin, remove that check, and stick it on a feature branch. (this Sun or Mon)
I'll commit the new branch and see if someone wants to merge it

Implemented on feature branch keep_build_forever in repository https://github.com/jenkinsci/promoted-builds-plugin

Just requires someone to check the code and merge to master

this is a duplicate of JENKINS-6166

Included in 2.0