This has nothing to do with server side processing. I’ts simply a missing feature.
This is not available for Nanoflows. I think the validations in question are processed on the server and, since nanoflows execute in the client without any server communication required, this type of validation is not possible with nanoflows. However, you could perform these validations in your nanoflow and not complete nanoflow execution unless requirements are met.
In 8.2 you can call a microflow from a nanoflow.
This helps a little bit since now you can use one and the same validation flow to do the validation. The downside is that this will generate a roundtrip when doing the validation. Vice versa would be nicer: having a validation nanoflow and call this from your microflow.