Unfortunately I won't be able to get to it anytime soon, see the explanation here: https://groups.google.com/d/msg/jenkinsci-dev/uc6NsMoCFQI/AIO4WG1UCwAJ . If somebody is interested to work on it (e.g. as a part of GSoC), I might be able to help to breakdown the task and to design it properly
This case can be solved by setting a secondary owner with the same as PrimaryOwner rights. But for more flexibility it would be nice to implement item-specific security on folders (https://issues.jenkins-ci.org/browse/JENKINS-47595)