Hi Leon,
Unfortunately you cannot query Clarizen recursively in the context of business rules. However you can use a WR to populate the top Parent Link each time a new Work Item has been created. Having said that you still need to address existing work items as well as change in hierarchy.
Basically the solution is comprised of 1 WR and one CA to help populate the value for existing WI in the system. After installing the app you'll have to filter your view using the Top Level Filter and then run the CA on the respective projects (you can run up to 30 projects at a time).
The use case for changing parents can is prone to bugs and I'm afraid it's not going to be robust, therefore, I didn't try to build a WR to meet that need.
Good luck!
Tamir