I think I somewhat misunderstood your initial question. Yes, it’s somewhat related to the @RJO’s question “Big feature request on all cloud connectors”. We will have database dynamic (prompted) authentication at some point. Chances are it can be done for OAuth connectors as well, but can’t guarantee it.
At this point, if you can’t use the free edition and still need the dynamic authentication, then the only way is to build a custom application (in C# or Python) that would do the authorization and then trigger an EasyMorph workflow.