Imagine you have a twin pipe setup that joins again at the final exhaust box. One would just have an inline valve in, the other section, smaller pipework, a cat and a silencer, also an inline valve in.
Smallerpipework would have an inline valve closed allowing full flow / noise. Main pipe closed valve, would divert flow to the smaller pipework, open inline valve on smaller pipework allow flow through the CAT.
I currently just have an inline exhaust control valve 4", the engine still runs and drives with it totally closed, but its massivel;y sluggish and the backpressure won't even allow the turbo to dump, the turbo also spools rapid, I know this because it whistles like hell when closed and driving.
Your proposal would solve your issue, but would still leave you vulnerable at a road side test. Probably never happen for 99% of us, I'm just trying to make mine totally legit.