Installing remote drains in mufflers...thoughts?

My mufflers are cracked, probably because they aren't self-draining, and have no drains, and spent a dozen winters in Canada. This seems to be a common problem with this model boat.

I'll be pulling the mufflers out soon to be re-skinned, by a retired Tiara fiberglass guy who has done this before. I'll be adding drains, but won't be able to get to them once the mufflers are back in place behind the engines. I'll be getting back there to remove the mufflers, still not sure if I'll be pulling the risers off to go over the engines, or trying to squeeze around the side. It is very difficult to get back there, so adding normal, manual muffler drains is not an acceptable option.

I could run hard-lines from the bottom of each muffler forward to plugs, but they would be quite long, and have to slope continually. Not a simple solution if possible.

My engines already have central-point raw-water drain systems, all the drains are actuated by air lines from a single Schrader valve and pressure release. It seems most logical and simple to add another air-actuated valve under each muffler, so they will drain with the engine. I like that it reduces the chance of forgetting to drain the mufflers. I wouldn't be against another method of remote actuated valve, by pull-wire or electric, but this seems simple. DC lawn sprinkler valves are pretty simple, rugged, and widely available. But bulky and plastic.

I'm settling on a P.O. check valve or blocking valve, used throughout the machine-building industry. When air pressure is lost to the control line on it, the main valve shuts off. That protects machines and workers when air pressure is lost to a machine: all the air cylinders lock in place rather than falling by gravity or momentum or spring. These things are dead reliable, but not really intended to run silty, sandy water. They will only move once a year, so not likely to wear out ever. If they ever fail to seal, I don't seem to be getting carbon monoxide exhaust from my cracked mufflers, just raw water dripping at high rpms, and I can see it in my bilge. So I don't think a failure will be hazardous, just inconvenient dripping of raw water. Catastrophic failure with exhaust flowing into my boat is very unlikely, and I have multiple CO2 sensors and have tested them.

These are two brands I'm looking at, 3/8 NPT thread, brass with stainless internals and Buna-N O-rings. Both under $50. The original valves on the engines are north of $300 each, and much larger. I prefer the one-piece Parker (silver, nickel-plated). The schematic is upside-down from the orientation I'll need to use. The inlet seals with an O-ring in compression, by spring force. It is opened by a smaller cylinder using a smaller O-ring seal. That sliding seal will be on the bottom in my install, and will get water and silt passing it in use, could end up with some silt in that O-ring, but not so much in the compression seal that stops flow. A leak in the sliding seal would have little impact other than it might lose air pressure and close sooner. Both are good for much higher pressures and temps than will be encountered. The brass threaded adapter is what I'll have added to the bottom of each muffler.

Thoughts/suggestions/warnings?
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Over 100 views and nobody has bashed me, I guess I'll carry on. Or maybe I was just too long-winded for anyone to read it all? I'm putting these air valves in the bottoms of my mufflers, so I can remotely open them with air pressure, to drain the mufflers.

I'll think about that option some more tomorrow as I'm removing them. I'll put the threaded bungs in for sure, and decide if I can run hose or need these remote valves. The center bilge isn't accessible behind the engines, so I'm skeptical.

They are below the waterline. I did think about something like that, but they probably need to stay somewhat full of water in operation to muffle. The side-exhaust is a bit of a joyous roar on a plane, much quieter at lower speeds.

I might just put ball valves on them. Now that I have them out, I see they have drain plugs, but they look a bit home-made and I doubt they were used much. I'll have the threaded brass bungs fiberglassed in with a fair amount of extra structure around them. I can make a stick with an end to fit the handles, and maybe put bigger handles on the valves.

No brass? Is that a salt-water thing? I've seen nothing but brass exhaust on most old mahogany boats, it seems to hold up. I can switch to stainless simple enough.

I'm not buying it, just trying to improve things, no surveyor involved so far.

Hmm, ok, no brass in my raw-water systems because it contains zinc, which will act as a sacrificial anode and dissolve.

https://www.proboat.com/2012/07/beware-the-brass/

Ordered all in stainless. The valves have small flanges for bonding, that might be overkill, but I want to do this right.

Well, I need drains. The least I can do now is glass in a couple bungs like the brass one in the last picture. Then either plugs, or valves, in those bungs. I just stopped at the fiberglass guy with my bungs, explained that I need structural strength on those bungs. He's going to build up fiberglass around them, thick and wide. He expects the stainless will bond well if he cleans it well, he's quite familiar with this stuff. He was a fiberglass guy at Chris Craft and Tiara, I trust him. If it does crack off, it will still be a pretty small hole, not catastrophic. I'll be able to plug it with something.

Rod says these were made in sections and epoxied together, but the type of epoxy and thickness at certains spots didn't hold up at the seams. The existing drain holes are just holes, no threads or anything. Probably not stock, but I'll use them.
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