Sounds like a good addition to Solr.
A document transformer would be the better choice. See:
https://cwiki.apache.org/confluence/display/solr/Transforming+Result+Documents
The primary purpose of a function query is to affect relevance, although,
yes, they can sort of be (mis)used as document tran
Has anyone written a Solr function which will reformat Solr's ISO8601 Date
fields and could be used to generate pseudo-fields in search results ?
I am converting existing appplications that have baked-in assumptions that
dates are in the format -mm-dd to use Solr, and tracking down every
plac