Drupal 7 Node Revisions per Role
I haven't tested it, but there is a relatively new module with a stable 7.x release that looks like it was built only for this purpose: Node revision Permissions Per Role


  Node revision Permissions Per Role allows to give roles administer per content type node revision permissions without giving them full administer nodes permission.
  
  For instance, you may let certain users(eg: roles:External-blogger ) manage node revision of a specific content type(eg: blog) but restrict to manage node revision of all other content type that this module is perfectly works for you.