You'll be spinning the pump backwards. The impeller is directional. So that would not be a good idea.
If you run the belt this way
__________>
O
it will spin the pump clockwise.
if you run the belt this way
O____________>
it will spin the pump counter-clockwise. But FYI, I've got the ribs of my belt running face down over a smooth tensioner. It hasn't developed any problems yet after a few thousand miles.
Check under the hood there should be a belt routeing.