I personally live in QLD so I am not familiar with interstate transport department rules. However I know that across the board that a modification plate isn't worth the metal they're made from, if the item installed isn't genuinely up to ADR specifications.
For example take this scenario:
My good friend "John Smith" has a welder at home, along with some off cuts of tube steel, and "Bob Stapleton" is a automotive engineer. Myself and John decide to make a set of rails to install a set of fixed back Recaros into my R32 GTR. We finish making the home made rails and take the car around to old mate Bob's workshop. He makes us a modification plate and rivets it to the firewall.
Honestly if "officer Leroy" was to seriously check these rails, and seats, they would never in a week of blue moons seriously meet ADRs. Some times the money you save on seats you will have to spend to bring their instillation up to scratch, and that's not even considering the value of your time and effort.
Ultimately what I am saying is that I would personally rather argue in court, if needed, that my new seats are a newer design factory Nissan seat, on factory Nissan rails, bolted to my factory Nissan floor with no need for modifications.
The R34 GTR seats are a fantastic bolt in upgrade to any post R31 model Skyline seat(s), bar the R34 GT-t. I am 6'2" and these seat are just perfect for me, they are seriously the best seats I have sat it. I have previously had sets of R32 & R33 GTR seats and as great as they are, the R34 items are still a couple of notches above them.