Here is my prediction.
The OIC will ban NR guns, as AR15 is restricted by legislation. The AR15 ban will come later.
The ban on the NR rifles will be to effectively close the tap on sales immediately. They will declare an immunity period for owners claiming they need time to structure and legislate on the buyback program.
By banning NR rifles, they can never be held accountable to an actual number of firearms that must be turned in and bought back since no registry exists.
They also do not have to revoke registration certificates, making the appeal process described in the OP irrelevant.
Wendy will be happy. Most gun owners of the affected firearms will keep them, some will get caught with them over the next 2 decades. These will be processed through the justice system, business as usual, and a few headlines along the way about assault weapons being taken out of circulation thanks to the liberal gun control move of 2020.
Then a few more shootings and madmen will keep the gun control flame alive and well, until the next round of gun control proposals.
All of this will insure Liberal votes for many terms to come, and gun owners will go about their business because banning 12 guns wont alienate enough people nor have any type of significant impact on the gun community and industry to yield a revolution. Some noise will be made, but it will be just noise.
Youza.