I'm working on searching Solr indexes created by Nutch at the moment, and mapping this where possible to the eZ Find schema.xml. As eZ Find is very much focused on querying/indexing eZ Publish sites and the more I get into it, the more I think creating a seperate search extension is the right direction to go in.
eZ Find 2.2 will have native support for querying other indexes along the ones for eZ Publish
In order to be able to do so, there will be common schema elements to query over and for the eZ Publish index there will be a few common/aggregated fields in addition to the default text we have now (and used in the simplified standardhandler now).
For a "foreign" Solr index, you may need to define some extra copy fields for it
