I did exactly that, starting here in my build thread.
The wall is framed with flat bar and expanded steel sheet, and has 2" of XPS on the back side. I built my own door for it which is hinged so it opens forward into the cab. I added windows on each side of the door and one in the door itself, salvaged from emergency windows from a lower-ceiling bus (thanks @inhof009). This gives more light and ventilation to the living area, and I can still see out the back through the child safety mirror while driving.
I obviously think it's a perfectly sensible thing to do, not just for insulation (which is the primary reason I did it) but also because it provides the driver (me) with some protection from things flying around the living area in the event of an accident. I think the reason few people do it is that they want to include all available space in their living area, and they don't want their living area to be so isolated from the cab because they have passengers riding in the back while traveling.
Another side benefit is that I'll be able to lock the bulkhead wall door if I ever have to leave the bus at a mechanics' shop for any length of time.