External Tool configuration - how to get subtree path Macro from Project window

Hello,

I'm trying to configure CheckStyle so that I can run it on any subtree
of my project that I select in the Project window.

If I just want to run it on the current file in the editor, I can set
the 'Parameter:' field in the Edit Tool's dialog as follows using
$FilePath$

-cp %CLASSPATH%;C:\checkstyle\checkstyle-3.4\checkstyle-all-3.4.jar
com.puppycrawl.tools.checkstyle.Main -c
c:\cvsTree\code\templates\MyCheckstyle.xml $FilePath$

My question is, how can I set the 'Parameter:' field, so that when I
invoke a right-click context menu in the Project window, it will run
CheckStyle on the selected sub-tree.

Thanks,

Ted