Thanks to the SortedSetDocValues this is in fact possible -- in fact i just uploaded a patch for SOLR-2522 that you can take a look at to get an idea of how to make it work (the main class you're probably going to want to look at is SortedSetSelector: you're going to want a similar "SortedDocValues proxy" class on top of SortedSetDocValues -- but instead of picking a single value, you want to pick your new synthetic value based on your custom function logic.
https://issues.apache.org/jira/browse/SOLR-2522 : I have a requirement where i want to maintain a multivalued field. However, : at query time, i want to query on only one value we store in multivalued : field. That one value should be output of a custom function which should : execute on all values of multivalued field at query time. : Can we write such function and plug into solr. -Hoss http://www.lucidworks.com/
