(know this is an old post... but what the heck)
When I've tried using Boo's slicing, it's never worked, don't know that the feature has even been implemented yet.
However, you can use the methods of System.String:
http://msdn.microsoft.com/en-us/library/vstudio/system.string_methods(v=vs.90).aspx
I think with a combination of IndexOf and Subtring (etc.), you can achieve what you were out to accomplish.
-Brian