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.
There is a complicated formula for calculating cooking time based on energy input, surface area, thermal conductivity of food etc. Smaller pieces with a larger surface area will cook slightly quicker than large single pieces
In general for roasts (large slabs) just add the weight of the pieces and cook for the minimum time recommend by your pressure cooker for that (or from your experience). Then test using an accurate thermometer and cook some more as required. In my experience most people overcook meat anyway, so try it a little more undercooked for a change. Remember to let it stand at least 5 minutes per 500 g too
There are too many variables to give a blanket answer (stove energy, pressure in pressure cooker, meat type, fat content etc.)
Best Answer
If you are using the wrong kind of meat (= tender meat low in connective tissue), the pressure cooker will make it much worse. With this meat, the more energy goes into it (and the pressure cooker gives it more over the same amount of time), the worse it gets. You really have to prepare it as a steak or roast.
It would be an interesting thing to try out on the right kind of meat though, since there you want to break down the connective tissue, and the pressure cooker could be better at it.