When you restrict the inlet flow to a pump you can cause cavitation to take place, which can eventually destroy the pump. Also, the little pumps we use for aquariums don't generate much, if any, suction. The flow rate just drops down when you restrict the inlet flow. It may be that the labyrinth you made won't restrict the flow much, so it would work fine, as designed. The only other possible problem I see is CO2 bubbles floating up out of the inlet instead of being transported down to the pump. But, if you ran it with the pump at the top, that wouldn't be a problem either. That's what I would do.
This might work well for those who don't like to have CO2 mist floating all over the tank, but still want to use a powerhead as a reactor.