Not much to add, other than I’m supportive of this. I feel like in the old Infopath world, this kind of thing might have been possible by persisting the HL7 message being generated by the form engine in a draft table that would allow the contents of the encounter to later be edited before finalizing and moving to a queue for submission, so my first reaction would be to try to do something similar to that but within the context of moving to FHIR and having OpenMRS support natively the idea of FHIR messages being constructed and in draft format before being finalized and submitted.
I do not think that it is likely to be doable / preferable to just add a “draft” status to the existing encounter table or Order table. This feels like a recipe for misinterpretation and a backwards incompatibility nightmare.