It's unfortunate how close this all seems to doing something really useful but then not actually getting there. I wish you hadn't dashed hopes by saying it's not on the roadmap (why on earth not?)
Attribute rules in gdbs are really powerful and really excellent features, and it would be great if they carried over to AGOL. It's more and more common to pass out data to field staff and have them edit live feature services, and if the less they have to input, the better. Being able to calculate fields based off other fields is awesome, but from what I'm gathering that's not actually possible. The linked Story Map deals with cartographic expressions only.
My hope was that Smart Actions would be a sort of attribute-rules-lite that would help us achieve this, but judging by your response that's not the case.