U3 / WBA. privileged read/write region that is readonly otherwise, without using MPU ?

The issue feels really simple, and it's easy to do on N6.

At the core, I am running out of MPU regions as there are only 8 available. So it would be nice to have a small chunk of SRAM, where privileged code can share data with unprivileged code. Say a status or something like this.

GTZC only controls both read and write. So I cannot say that a single 512 byte block is read/write for privileged code, but read only for unprivileged.