Unless you are prepared to build some industrial strength equipment of your own design and then move everyone in the neighborhood away while you experiment with this, I fear you are taking your life in your hands.
Normal pressure cookers add a maximum 15 PSI to achieve a water boiling point of 121 C or 250 F. Autoclaves, used for surgical sterilization, go to 30 PSI. You are talking about going more than twice that.
There is no reason, based on the science of Maillard reaction, to believe that it would not occur at a high enough temperature. The presence of excess water would normally inhibit the process because of temperature reduction, but your "super duper pressure cooker" would keep the temperature at a high enough level to allow the chemical breakdown to occur. You might, in fact, discover that it occurs a bit earlier, as water tends to facilitate many reactions. Caramel making comes to mind as an indicator of what might be achieved, as sugar syrup (OK, most of the water is gone, but in principal) browns when you get in the above 330F-165C degree range.
As to crisping based on quick pressure reduction (perhaps when your device explodes?) That seems less likely as most crisping comes at the loss of water, and you are, in effect, keeping water in contact with your food both in liquid and superheated steam form. It would, most likely, be similar to a braised food surface, than a fried one.
Interesting thought. Please don't try this.
Try making braised pork chops with onions. Salt and pepper then brown chops on both sides in a heavy frying pan or skillet. Remove chops, add a little oil if needed, and add a sliced onion, stirring occasionally until the onion is lightly browned. Add water or chicken stock or a combination to the onions to deglaze the pan, then nestle the chops back into the onions, cover and reduce the heat to a very low simmer. Cook this way for about a half hour to 45 minutes (test for fork-tenderness). Check occasionally to add more liquid as needed and to turn the chops.
Best Answer
Nope - this isn't really true, nor is it a complete description of what's going on. Pork belly doesn't just contain a large amount of fat, it contains a large amount of connective tissue (which is why it's so tough when not properly slow-cooked). The goal with such cuts (ribs and pork shoulder are other good examples) is largely to break down tough, chewy collagen into gelatin, which provides a deliciously viscous texture. It's sometimes described as "mouth-coating", which sounds... less desirable than it is.
Both the process of melting fat and the process of converting collagen to gelatin are temperature-dependent and will speed up with increased temperature. This is the advantage that your pressure cooker provides. However, the latter process is a lot more complicated; it requires the presence of sufficient water, and it doesn't scale linearly (e.g. it won't get 10% faster by applying 10% more energy).
There seems to be an upper bound with how quickly you can get collagen to convert to gelatin, and with pork belly there's so much collagen that a typical cooking time might extend into 12+ hours. And that effort is necessary, unless you want a chewy pork belly after all that work. With a pressure cooker, you might be able to get the cooking time down to 9 or 10 hours, but the reduction won't be as drastic as with other foods.
Additionally, gelatin itself is temperature-sensitive, and pure gelatin will break down if boiled; what seems to happen is that the long, flexible strands that give gelatin its elasticity get "cut" at high enough temperatures, such as the higher temperatures in your pressure cooker, and it loses some of that lip-smacking goodness. I've observed this myself when making stock in a pressure cooker. It's faster and more convenient than letting a pot simmer on the stove all day (and the endless skimming, and topping off with water, etc.) but it never has quite the texture of a stock made the "proper" way. Don't get me wrong: it's still quite good, and pork belly has so much collagen that you might not notice a difference in the end, but there's an argument to be made for accepting the additional couple of hours for a traditional slow braise.
So, the TL;DR: a pressure cooker will indeed speed up the cooking of pork belly, but not as much as it speeds up other cooking.