Shaw520, Is the 520 a KTM, how hard is the clutch pull, and is vibration a problem? Just curious as I am waiting on a 520 EXC.
BTW: Aspirin is not a bloodthinner, it is an anti-platelet agent, Coumadin acts by inhibiting formation of clotting factors. Neither of these drugs will increase blood flow (except on the ground)or reduce arm pump but both will interfere in your body's attempt to form a blood clot. Not what a motorcycle rider needs!
The best explanation I have seen for arm pump, is inadequate oxygen supply to a muscle bed and resultant anaerobic metabolism and lactic acid build up. Arm pump is worse before we have completely warmed up with increased heart rate and cardiac output, and dilated our muscle bed. This means a good sweat, not just jumping up and down a few times. After the initial arm pump, it usually goes away for the rest of the ride. By then we have relaxed the death grip on the handlebars and the heart, lungs and muscles are warmed up and working efficiently. I would be very surprised if there is a powder mix which would help.