I was considering that my understanding of tri and quad laminate suits one of the layers is a semipermeable membrane that allows 'sweat' out but prevents water intrusion to varying degrees.
My experience with that is surface weather wear, repairing leaking seams and holes, and to a lesser extent water filtration. (... and kidneys)
I thought, perhaps in a cost saving measure there were different membranes with differing ability to resist intrusion at pressures. Perhaps they are lighter and more flexible, while ones that survive at multiple atmospheres are thicker, heavier and inflexible. (Again, I don't know what I don't know).
Perhaps it is more correct to simply say water pressure resistance, but I felt like the membrane was osmotically allowing vapor to go one way and resist liquid from the other, and so, described my notion.
You considering a wrist dump gives me a little hope. (I thought about one at the ankles, honestly). My impression of most divers are that they don't really enjoy custom or modified gear, and so they never really considered what was different in a high end surface suit versus a low-tier diving suit and simply ponied up 3 grand... buy once cry once?
I thought, punch a hole, then glue the layers, then apply tape or rubber to dress the edge of the circle.
I'd be skeptical. There's a thread on this that seems to suggest breathable tri-lams aren't as robust:
Scubapro's Evertech Dry Breathable Drysuit: Your opinion please...
If the suit works like Gore-Tex, it's not based on osmotic pressure but simply size of opening: The pores in Gore-Tex are small enough so water drops are excluded, but large enough to allow water vapor to escape. In theory. It's just size-exclusion, not osmotic at all.
On the other hand, if you're diving salt water you might get an osmotic pressure between sweat and seawater (sweat is more dilute than seawater). So a membrane that allowed water through but excluded salt would tend to let water leave and keep the salts trapped inside until the water inside was the same concentration of salt as outside. In theory, with lots of caveats. (E.g., if the air in the suit is dry, that can reverse the flow and pull the water from the ocean into your suit.)
And this would fail miserably in freshwater, because under all conditions water would want to flow in to the suit due to osmotic pressure. All to say I don't think osmosis is involved.
On the dump valves: I don't know if it'd work for you, but the Apollo suit (BVS 4.0) I mentioned comes with a standard shoulder dump, left wrist dump, and ankle dumps. The wrist and ankle units can be set to "auto" or locked shut. I tend to recommend them to novice dry suit users because they're inexpensive (relatively) and the ankle dumps add a measure of idiot-proofing that might keep them alive to dive another day. (Yeah, they shouldn't dive a dry suit if they NEED ankle dumps.) The neoprene wrist and neck seals are also a little less finicky than latex or silicone seals. The attached boots are more rugged than any other I've seen, which is good on my rocky coast.
Before you think I'm on commission with Apollo (I should be), I'd note the downside. They're neoprene and ultimately neoprene will fail in such a way it's not worth fixing. Just like a wetsuit. The other downside is that if you somehow puncture it (more likely as it ages) it's not easily fixed in the field. I no longer dive my Apollo as I got a deal on a high-end DUI suit, which has field-replaceable neck and wrist seals (or dry gloves). I suspect the DUI suit will outlive me, but if it doesn't I'll probably go back to an Apollo because I'll be at the "old enough I don't buy green bananas" phase of life.
And for any other manufacturer, it's very possible to put dump valves in wherever you want them. Pee valves are similar: A lot of folks add them to a suit post-construction.
