I dont own a HPS or MH all I have for now is Florescent and CFL.
Yes you could run more lights. A plants natural instinct will be to streatch toward the light. So this would make the plant grow sideways NOT UP.
in therie this would give you a hanging garden effect as well as every branch that came out the side would in therie be a top.
MUCHO GRANDE' NUGGS!
But this all has yet to be tested.
I will have mine built after the new year. I need to take clones first.
Also if your limited in space see my thread called Micro Ebb and FLOW.