Yeah, I've thought about this, but I think you need more than one degree of freedom for the chair to help with motion sickness. Like, if your character is in a car and accelerates, you need to tilt (pitch) backwards a bit, to emulate the way the acceleration pushes you back into your seat on the car (well, really it's the corresponding motion in the inner ear we need to worry about, but a tilt is the correct solution for both). When you go around a corner, it needs to tilt (roll) sideways a bit, to match the feeling of being pulled to the outside of the turn by centrifugal force. Etc. Those are the forces our inner ears are expecting, and without those, there's still a mismatch. And even with the hardware to do those movements, you need software to calculate the right motions ahead of time so you can reach the right positions in time to match the visuals, which is also quite difficult, and makes it pretty hard to picture doing this as a peripheral rather than as an integrated system. And the cost would be prohibitive.

Honestly I think we may not get this until we can fake it all with electrical signals to the brain, which is quite a long way off.