Hello,
unfortunately, Lucene / SOLR search can not be used for wWorkflow queries because workflows and tasks are not indexed by default. The only way to query in standard Alfresco is via databse query, and this is what the workflow service does. Unfortunately, this means there are some restrictions in place regarding the complexity of a query so that the execution does not take ages and can be expressed in dynamically generated SQL (via statement templates).
Your best bet in your case would be to build a service facade / web script around workflow service and execute multiple single user / group queries to aggregate all relevant tasks / workflows, or query all tasks / workflows of a specific type and apply a post-processing filter to the results.
It is theoretically possible to search workflows / tasks via SOLR / Lucene by hooking in some low-level replication / duplication logic so the data they contain is represented via regular Alfresco nodes and thus picked up during indexing / search. We did a proof-of-concept for this internally and it is working quite well. It is of course a pretty significant change within the core services of Alfresco.
Regards
Axel