Well, at last the jump was made, and based on the previous 2.9.x branch, the migration to Lucene 3.5 has been done with version 3.5.2 now available in central. Apart from the migration work, the most significant changes include:
Writer configuration suppliers, to deal with the fact that certain Lucene objects, such as merge policies, can't be shared.
Managed readers, not based on the new searcher manager, but integrated with the lucis store architecture.