I've been doing some research on doing a search of the user base, using profile values for querying. I know there's a
sitecore_core_index which in theory would cover that, but I'm guessing it's not that easy.
In looking around, the best I could find was this: https://gist.github.com/jorgelusar/150a607dcf96acb54b60. The associated blog that popped up in Google was no longer available, so this is all I have to work from. It looks like it would do the trick, and the code/config would just need to be modified for Solr (instead of Lucene).
My one concern is that right now, it appears to scan over the whole user base to do the indexing, so that it either has to be kicked off manually or run on a schedule. It'd be ideal if it could run when a user's information is added/updated.
If someone has ideas in that department, or knows of a more recent/modern (9.x) blog that discusses this subject, please point me (and all of us) in the right direction.