During inverted banking, what happens to heading?
Would a plane in 135° roll be using stick-forward to retain altitude? And thus bank in the direction the wheels are pointing?


Yes and yes.  

Let's say you're flying North.  You roll inverted, then lift your left wing 45 degrees above the horizon (this is the wing that pointing East, now that you're upside down).

You'll experience the usual G forces pushing into your head (it hurts), and the plane will start turning to the West.

I once saw a great diagram that explained which controls do what when inverted - it showed the outline of the plane with one cartoon pilot sitting in the normal position, and another "phantom" pilot sitting in the same seat with feet on the rudder pedals, holding an extension of the stick, but with his head pointed the opposite direction - head towards wheels instead of away from wheels.

When the plane's inverted, the "phantom pilot" is upright, and all the controls would behave normally from his point of view.