Maillard in a Pressure Cooker
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.