No, there is no way to do this in any version of Project Server, including the 2003, 2007, and 2010 versions. The only way to restrict PMs from editing only certain tasks is to break up the project into smaller projects. One project would include tasks 1-20 and the other project would include tasks 21-40. Project Manager B would need set him/herself as the Status Manager for the tasks in both projects. If there are links between the tasks in either project, you could create a temporary master project including these two projects as subprojects. In this master project, you can set the cross-project links as needed, then close the master project without saving it, and finally save the subprojects. Hope this helps.
Dale Howard, MVP VP of Education, msProjectExperts