Here's one idea: have an exhaust shop make up some "Y" pipes to put behind your headers. One branch would go straight to your existing pipes and the other would just dump. The second branch would be covered off by plates held on with wing nuts. If you had everything made from stainless pipe with stainless hardware, it should last a while.
Here's the only good "automatic" idea I have. Use a pair of stock "heat risers" to block off the straight exhaust path. You would have to modify them a little to make them close all the way (instead of just most of the way) Then you could use the stock vacuum actuators to open and close them. Ideally, use would use vacuum to open with spring close, but they are set up the opposite way. Maybe that could be changed? The diameter of these isn't real big, though. Mid-80's trucks have a vacuum switch that is electrically operated for the EGR valve. You could snag one of this to give you control over the system with a switch.
I think reliability will be the biggest issue. Good luck