My big block HQ Van was originally engineered in NSW with a bench seat in the rear (pointing forward) although it was stupid and I pulled it out.
To get something engineered you must draw a comparison to another already ADR approved vehicle. This gives the engineer something to base conclusions upon.

Ultimately suggesting the use of a dickie seat from a EL Falcon (7 seater) wagon or similar should see it past the post. Even if it is only suitable for children to a certain age.