I had the same thought but then I realized how many exhaust thru-hulls are that crappy white plastic and I've never seen one fail from heat damage.
The one on my boat is heading for 33 Y.O. and is still solid.
Yeah my present through-hull fitting is a 26 year old white plastic mushroom.
Guess I'm just wondering if long-term exposure to heat and/or exhaust gasses would be a problem for a seacock mechanism (plastic or metal).
Or this just a dumb idea altogether? Just stick with a plain barbed through hull and shove a plug in it to change the exhaust hose?