A couple notes. I think there's a detail you aren't getting about the magic of steam to transfer energy compared to water (or metal, sand, or most anything else). Latent heat of vaporization/condensation. Basically, you know it takes a little bit of energies to make a drop of water a little bit hotter. But it takes a whole bunch of energies to boil that drop? That's the latent heat of vaporization. Well that process works in reverse, LHC. When the steam hits the inner pot, it condenses, and that transformation instantly transfers all the energy that it took to boil the water. It's the magic of phase change, and it's essential to steam jackets being workable and bain maries not.
Btw, "latent" heat is opposed to "sensible" heat. Sensible heat you feel, like it's 10 degrees warmer in here. Latent heat isn't "felt", steam can transfer incredible amounts of energy and hardly change temperature. Sensible heat scorches the pot, latent heat mostly just transfers energy.
On the pressure, most commercial units are built for 15-30 psi. I have one, and was pretty surprised to see it never really got up to pressure even though the gauge goes up to 30. But,
all the energy of the steam condensing on the inner pot just powers the phase change inside the wash, and that energy is carried away to the condenser. The process is efficient and quick, so energy is constantly leaving the pot so it doesn't really need to / able to build up high pressure. Which is the whole point behind steam kettles, steam engines, seam power plants etc, steam is really efficient at transferring energy.
Sorry I can't give exact figures, because after figuring it out and verifying my pressure limit safeties work I pretty much never look at the pressure because that isn't how I drive it. But I think you could get away with 5 psi. Maybe even 2 psi, although at that low temp the last part of the stripping run may slow down a bit, the difference between steam and kettle temps does still affect energy transfer. But it will still boil, 2.3 psi steam is 7*F hotter than bain marie style boiling water, and has all that latent heat too.
Which may be encouraging because I don't think you'll have any hope of a soft seal removable deal holding 15 psi. I'll be impressed if you can hold 5 psi, that's about what open head steel barrels with clamped lids can handle, and any setup I can imagine for you doesn't have that kind of mechanical rigidity. And honestly, I wouldn't want to be anywhere near a homebrewed pressure vessel. Even at just a few psi, there are huge overall forces, and potential for cataclysmic forces if you things fail badly. Please think of every way to do this safely, and double up whatever safety measures you go with.
