Has it ever been tried ? As arm pump for racers is becoming more and more prevalent and a limiting factor for their performance. Would a more vertical control package mitigate that ? Do clip ons have to be straight tubes ?
Due to our riding position there is always a vertical force on the bars, I would think that having a vertical bar would be quite tiring to hold on to. Horizontal bars are easy to hold on to because of the vertical force being perpendicular to them.
IMO hand/wrist/forearm orientation has little to no affect on arm pump. I have a lot of experience with arm pump from rock climbing. The direction that you pull or hold onto climbing holds doesnt change the pump at all. The major factors that do affect pump are training and how tightly you grip. So I doubt that a different bar position will aid anything for your avg pro racer. Because of climbing, I never experience arm pump at the track. My forearms or hands may get tired, but I never pump out. I'm also not racing in GP. I am surprised that we haven't seen GP racers publicly incorporate climbing into their training, especially since the sport is much more popular in Southern Europe.