Ok, The McGyver in me just had to know if you can really sand down a chip to that sort of thickness.
So I grabbed a dead MZX7219 and some sand paper, and... yes you can!
I easily got it under 2mm thick (actually 1.85mm) without exposing any silicon. With a 0.8mm PCB that meets your "under 3mm" goal.
It's probably not a good way to do it if you want to make a lot of them, but it can be done. 