This is now fixed! I just committed a new query, ToChildBlockJoinQuery, to perform the join in the opposite direction. I also renamed the previous query to ToParentBlockJoinQuery.
You use it just like BlockJoinQuery, except in reverse: it wraps any other Query matching parent documents and translates it into a Query matching child documents. The resulting Query can then be combined with other queries against fields in the child documents, and you can then sort by child fields as well.
Using songs and albums as an example: imagine you index each song (child) and album (parent) as separate documents in a single document block. With ToChildBlockJoinQuery, you can now run queries like:
albumName:thunder AND songName:numbor
albumName:thunder, sort by songTitleAny query with constraints against album and/or song fields will work, and the returned hits will be individual songs (not grouped).
ToChildBlockJoinQuery will be available in Lucene 3.6.0 and 4.0.