Let the Build system author evaluate shell commands before they are run. This allows her to add variables like %SOME_SPECIAL_VARIABLE without interfering with PBS.
%PBS_REPOSITORIES is handled in this plugin. The default handling replaces %PBS_REPOSITORIES with the path to the node to be build in each source directory.
Special handling of repositories and include paths is left to the user (thus this user modifiable plugin) as there is no consensus about which of those has precedence. building object nodes through a perl sub is another solution where the user has total control.