In my opinion portlet filter should work the same way like servlet filters do:
An example: If you have a filter chain with filters F1 and F2, then the chain is: F1 -> F2 -> target -> F2 -> F1.
An exception is, if a Filter does not call filterChain.doFilter. Then no other filter or the target is invoked and the filter itself is responsible for the response.
(e.g. see http://java.sun.com/products/servlet/Filters.html)
In the current implementation the target is invoked, no matter if a filter blocks the chain or not.