The problems with valves is that they broke down easily, especially with bad wheater or close to the sea.
Even if you go with the Pierburg ones, that are made in Germany and not in Taiwan or Chine like the ones you find on many aftermarket exhaustm, they'll still break down some day.
You could easily design a system with a valve somewhere after the cat, there's not much room on the 172 and it's a drony engine so it'll be probably better to make a new backbox with a Y pipe before it and twin pipes inside the muffler, a drilled 54 mm pipe and a 63,5 mm solid pipe. You'll have two stealth exits behind the bumper, like you see on some 2 liter diesel cars as there's not enough room to merge them after the muffler.
With the valve closed the gas will exit from the smaller silenced pipe, and you'll get a good noise even with a sports cats or a decat pipe (i'm running this configuration on my car with a centre 63,5 mm silencer and the noise levels are spot on for daily use), then with the valve open the gass will mostly pass through the bigger solid pipe due to the lower backpressure and you'll get a very louder sound (even louder that the backbox I was using a month ago, with a drilled 63.5 pipe)
The only problem with such an exhaust is that with a Pierburg valve that costs around 200 euros, plus the custom made backbox with a y pipe, the wiring or wireless control of the valve you're looking at 1000 euros catback system, and I don't think anyone with a 10 year old clio would spend that much!
Also, there's the problem with the weight of such a system, the standard hanger won't be enough.
Something like this (sorry for the roughness of the photoshop job)
On the 182 you could arrange a similar design as you need a big backbox to have ad acceptable noise when cruising at medium-high speeds, obviously with a transversal muffler.
that and a different backbox could work 
