Search API Users filter on roles? While I can filter on content types in Workflow / Data Alterations I do not see anything for roles when adding users. Is it somewhere else or is it something needs to be developed? If the latter, where can I find some pointers, examples on how?
At least from my point of view the API of search api is pretty good documented in the .api.php file. There is a plugin type called "callback" which allows you to filter out stuff with configuration. This and the callback_bundle_filter.inc should give you enough to provide a patch :)

And don't forget to add your include file in the .info file