Believe in general it would be a nice feature to be able to overwrite the FieldCache-implementation in Lucene.
For instance we override with a FieldCache-implementation that basically does not allow anything to be put in the FieldCache caches. We have so much data that it always creates memory issues.
But I could imagine other reasons for wanting to override the FieldCache-implementation
Doing it for Lucenes under Solr, it is obvious to do it through solr.xml (and not solrconfig.xml, because FieldCache unfortunately is per-JVM/classloader and solrconfig.xml is per-core)