barometric dampers are designed to regulate draft, or negative pressure. On positive pressure they close. I guess you could attach one to the return as that is a negative pressure environment. What is your static pressure on the S/R. Your furnace is rated at .5" static, probably. Perhaps you could achieve your results by going with a lower MERV rated air filter this time of the year. What is the MERV on your filter? Higher number is tighter, meaning more resistance to air flow. I think I'd do this before anything else.

With an incline manometer or dial manometer or digital for that matter, place the tube perpendicular to the air flow just exiting the furnace and note the reading. Do the same on the return. Add the numbers. If it is .5" or less you should have sufficient air flow at that point. Now read it on the system side of the filter. Read it at each change of direction on the S/R and you can see if you have nay major restrictions. Check the ductwork out before modifying anything.