Posted by Instant Flow steps in Business Process Flows are now in preview, yes!
See the announcement on the Flow Blog: Preview Instant Flow steps in Business Process Flows
This is/was the missing link I was referring to in a blog post I wrote:
Addendum
With the August 22, 2019 preview update, you can now mark instant flow steps as complete/incomplete, right from your flow!
See Flow blog post about product updates: Preview Update: Instant Flow Steps in Business Process Flows