Back spacing is a simple measurement from the mounting surface of the wheel to the edge of wheel. Wheel material, size or construction has no baring.
However, size of wheel will impact whether a wheel with a given back spacing will fit a given car.
The OP question regarding how much you can off set the center of force before it impacts bearing life I don't really know. I don't think you could enough on an A body before you would be hitting hitting body or suspension parts. Remember typically as the back spacing increase/decreases the rim width changes and the center of force stays the same. An exception I can think of is the old style chrome reverse or a FWD rim on one of our cars.
5" would be the max before the rim would be hard against the springs on the rear. I have 4.5" on the back of my car and with the tire bulge I have 1/2" of clearance to the spring. Up front the wheels are 4.25", I have never checked the clearance with the wheels at full lock to see how close they are to touching the inner fender.
It's only the off roaders that I have heard of wearing out bearings due to off set.