Technically this may be possible, but the big issue I see is that wireless charging is pretty inefficient. Only about 75% of the power actually goes to charging the battery, so if the battery case used QI to charge the phone then you would need to have a battery that was 1 and 1/3 times larger to get the same amount of power as you would from a battery case with a wired connection. In addition to the bigger battery, adding QI hardware to use to charge the phone and receive a charge I suspect that it would make the case too big and bulky compared to it’s wired competitors.