We have a PostFilter for ACL filtering in action that has a similar structure as CollapsingQParserPlugin, i.e. it's DelegatingCollector gathers all documents and calls delegate.collect() for all docs finally in its finish() method.
In contrast to CollapsingQParserPlugin our PostFilter is also called by the ExpandComponent (for purpose).
But as the finish method is never called by the ExpandComponent, the "expand" section in the result is always empty.
Tested with Solr 4.10.2.