I'll preface this post by saying this is not my preferred option or I think the best option for the new North Stand, I've also no idea the effect on sight lines from other blocks due to this idea.
Partial safe standing using the central blocks in the expanded North Stand Level 2:
The "General Admission" capacity of the 2 central blocks of the expanded NSL2 will be about 3100. Even if they don't make the entire tier safe standing, I think there would definitely be enough demand from relocaters/new season ticket holders to fill at least 3100 rail seats. If you include the "GA+" seats, the total rises to 3500.
I think the club could agree to this, with a mix of rail and normal seats in the North stand giving a range of options for people. 3000+ singers under the new roof designed for good accoustics could create an effective second home end and spread noise around the stadium from that end, it would also encourage noise to spread across the rest of the seated North Stand.
Ideally the whole second tier would be safe standing, but a 3000+ section isn't a bad compromise either, we know how vocal 3000 singers together can be from away ends. I just doubt whether there would be enough demand for 7000 rail seats in the North stand as it may be difficult to convince enough vocal fans to move from the South Stand, though I really wish I'm proven wrong.
If the demand is there, they could always expand the rail seating sections there to the other blocks either side if needed. I think if the club are serious about making the North stand a home end, it would be good to start surveying fans to get an idea of numbers and plan accordingly. This needs to be organised effectively.