It should be quite easy to prove that anything is more effective than 6-mil polyethylene sheet. Even a surface-applied treatment with a modest resistance to vapor transmission pressure will out-perform the code-required minimum–especially over time.
However, using a Class A below-slab vapor retarder costs much, much, much less than the surface applied treatments.
Legaly there is no such thing as a general approval of an alternate means of compliance. Each use must be approved by the local building department. The building official’s approval does not relieve the applicant of any liability. If the building official does not specifically approve this alternate then this deviation could be considered a code violation.
I am not aware of anybody who is recommending such a system for new construction.
The structural engineer should consider that if he uses an alternate means of compliance and something goes wrong that he will have more liability exposure. He is actively promoting this system as opposed to implementing a code requirement.
It is not clear what is driving the SE. Many structural engineers would consider the vapor retarder a part of the building membrane system and thus outside of their scope of work and thus happy to defer to the architect. What is the opinion of the Architect? Does he not consider the building membrane and issues related to moisture and water to be part of his primary scope?
At a certain point this is a place for a specification consultant to provide factual information to the licensed professionals signing the documents and let them accept the liability for their actions.