+1 for adding a big red flag. My application depends on this functionality, and it's working well once I overrode a bunch of additional methods in IndexReader that deal with Directory, IndexCommit, index version, etc.
(A few details on this, and why my solution is not applicable in general case: I'm using ParallelReader, and the other indexes that I add are throwaways, i.e. I recreate them on each index refresh from external shared resources. So I basically short-circuited those methods that deal with directory and commits so that they return information from the main index. This way the file-based replication works as before for the main index).