Locally Reinforce Drywall for Wall-mounted Seat

[kungfuu]

I have promised to supply a hall with a foldable wall-mounted chair. Although it's a lightweight wall, it's usually manageable by hanging in the studs and using snap toggle anchors. However, there are two concerns in this case:

1. It's impossible to find a single stud using any method, including a stud finder. All pilot holes were in vain. I still have yet to try with a magnet. Despite this, the wall is stable and feels relatively solid.
2. The cavity in the wall is only about 25-30mm with a wet room on the other side of the wall. Not feasible to pull anything through.

It's absolutely the strangest wall construction I have ever seen. Suggestions for local measures? I would like to avoid external studs from floor to ceiling. There should mostly be shear forces here but also some tension in the upper mounts.
4 attachment points in the seat in total.

 

[Tompafix]

I have promised to provide a hall with a wall-mounted folding chair. Normally, this is manageable with a light wall by mounting in the studs and using snap toggle anchors. However, two concerns arise in this case:

1. It is impossible to find a single stud regardless of the method, including stud finders. All pilot holes in vain. I have yet to search with a magnet. Despite this, the wall is stable and feels relatively solid.
2. The cavity in the wall is only about 25-30mm with a wet room on the other side. Not ideal for a through-mounting.

It is absolutely the strangest wall construction I have ever seen. Any suggestions for local solutions? I would like to avoid surface-mounted studs from floor to ceiling. It should mostly be sliding forces here but also some tension on the upper mounts.
4 attachment points in the seat total.

What is the wall's covering material?
A piece of construction plywood that you fasten with many attachment points, then attach the chair to this. Just paint it with a bit of color first.

 

[kungfuu]

Today just regular plasterboard, but as mentioned no studs found to attach to. Sure, it could work with an adapter plate, but many close fixings are a weakness in themselves.

 

[kungfuu]

After some testing, I think it might be possible to finagle some Snap Toggles from the back, that should do the trick. I will also check for studs/screws with a magnet.