I looked at SOLR-4465 and SOLR-5045, where it appears that there is a goal
to be able to do custom sorting and ranking in a PostFilter. So far, it
looks like only custom aggregation can be implemented in PostFilter (5045).
Custom sorting/ranking can be done in a pluggable collector (4465), but
this patch is no longer in dev.
Is there any other dev. being done on adding custom sorting (after
collection) via a plugin?