Polo Bikes

Could you mount the gripshift on the seatpost? If you put it right up the top it may be out of the way enough not to interfere with your legs. As I understand you don't need the shifter available during games.

Long answer yes.
You could mount the shifter on your seatpost is your seatpost was a 22.2 diameter, but that size i believe is reserved for the shittest of the shit kids bikes and nothing else.

Short answer no.