How to offer facet by group in Groups 2.x

Problem/Motivation

Using Group 1.x I was able to offer Search API, Search API Solr facets based on group. Nodes were indexed and it was possible to use the "Reverse entity references" processor to index a field like "Reverse reference: Group content > Parent group > Group > Title" as a string, then expose a facet allowing users to filter search results by group.

This doesn't appear possible in Group 2.x; during the group_update_9200 search api reported an error with the Reverse reference: Group content field, and again in group_update_9210. I've removed the reverse reference field from the index, and removed the facets.

Proposed resolution

If it is possible to continue to index group title from nodes, I'd appreciate some pointers. Also, if it's not currently possible, I can look at writing a Solr plugin to expose that info, but I'd appreciate some insight as to how to restore the functionality from 1.x.