In the Bob project (the project where we gather our projects aiding in the build and development process) we now added a cute little plugin that add a directory based class path container. The functionality is rather simple but, as far a I think, is missing in Eclipse for a long time.
You can add a new class path container entry to the project and define a path (using Eclipse variables) which will be used as a base for scanning for JAR files. All JAR files will be added to the projects class path. Also will the class path refresh itself when the directory content is changed.
The plug-in can be installed from the update site at http://repo.openscada.org/p2/bob/R using the Eclipse update system.