Originally Posted by toothy
Can you cut some of the green color wallboard and run a sheet of 3/4 plywood up the stairwell face of the wall, bonding the lower wall with your pony wall?
This would work, although 3/4" would be way overkill for the purpose. The ply would essentially be in tension/compression as that wall got pushed on, so 1/4" would be plenty. In fact I'd bet that re-running your drywall so it was continuous over that joint would stiffen things up significantly.
Depending on what's going to happen in that little space behind the half wall, you could add a little 6-12" L at the free end pointing away from the stairs.
And hopefully you're screwing that bottom plate down, nails won't offer much resistance to pullout as that hall gets pushed in or out at the top.