I would love to be able to use a trigger that you can specify one or more
specific directories inside the workspace to poll for changes instead of the
entire workspace.

The use-case is we have several ant targets that each build and test a certain
portion (but not all) of a project. And if we could have the SCM trigger set to
poll for changes in specific directories, then we could essentially make
separate jobs for each buildable subsystem, and only build those jobs when they
have actually changed.