Hi Jens,
that's easy.
You just need to create a specific ACL for the Dev Path and deny the
- checkin,
- modify member revision
- add member,
- drop member,
- add subproject,
- drop subproject,
- and configure subproject
permissions for the everyone groups
HTH Matthias
Note: Keep in mind that you ensure the ACL for the Dev Path is not inherited from the mainline.