Solving poor upstairs airflow with a fan-powered vent
The fan idea would increase airflow, but the fans would have to be small to fit inside the duct.  Small fans trying to move a lot of air means lots of noise, so be prepared to deal with that.  There are commercial products available that do what you're describing.  I tried one (http://a.co/8beWhs0) and it helped, but it wasn't worth the noise to me.  It was especially annoying when the temperature was very close to the set point of the fan's thermostat because the fan would turn on and off every few seconds.

I suggest that you replace your HVAC system's thermostat with one that lets you schedule the fan to run periodically throughout the day.  (The Nest thermostat has this feature.)  It helps to even out the temperature around the house even when AC and heat aren't being used.

An attic fan and/or additional attic insulation could also help.