If you are looking to fry veggies then yes, it is necessary to pre-heat. If you put them in a cold pan with oil, you start 'sweating' the veggies instead of frying them.
For example: if you put a sliced onion into a pan with hot oil, it will cook and have a nice golden brown color as it caramelizes. Put that same sliced onion in a cold pan with cold oil and then add heat, and the onion will first turn translucent and lose moisture.
Both of these cooking methods have their uses, but they are different.
How do you know when the oil is ready to cook? It will start to shimmer a little before it reaches the smoking point. Put the veggies in when you see this shimmer, or at the very first wisp of smoke.
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.
Best Answer
We do cook over 100C in a pressure cooker, but not enough to brown food, as this would take very high pressures. It probably wouldn't be safe or economical at home, and doesn't have enough of an advantage over ovens (which may use steam) for it to be worth doing industrially.
Solubility is a big factor, for example salt dissolves in water but not oil, and capsaicin (chilli heat) dissolves in oil but not water. Hydration is important to the texture of many foods (starch, for example). Cooking in oil reduces hydration, while cooking in water increases it, so I doubt you could make chips (fries) in water even at 200C. New techniques would need to be developed unless this extreme pressure cooking was just a way of speeding up existing methods.