Bypass maybe? If it would fit you could weld a bypass with a similar machenism to the throttle body. With that closed the exhaust will be forced to go through the cat and o2 sensor. When it is open the majority of the gas will take the bypass because of it having the least amount of resistance. You would probably have to have 2 different tunes though. One for when the plate is closed and another when it is open because the second one will have to not only take care of the better flow but also the lack of your o2 sensor's input to adjust A/F ratios. You could also set up the bypass like some chevy exhaust that has the two pipes welded to 2 plates and the plates are bolted together. Just stick a piece of flat metal next to the seal to keep the exhaust from taking the bypass.
