There have been requests for supporting multiple facet.prefix for the same facet.field. There is an open JIRA with a patch:
https://issues.apache.org/jira/browse/SOLR-1351 Wouldn't using multiple facet.query achieve the same result? I mean something like: facet.query=lastName:A*&facet.query=lastName:B*&facet.query=lastName:C* Bill
