The Maillard Reaction requires temperatures of 140 to 165 Degrees Celsius; hence "browning" cannot be achieved with water when cooking under normal conditions since the boiling point of water is 100 Degrees Celsius, therefore to brown food we use oil, or direct heat.
However at 10 atmospheres of pressure the boiling point of water is 180 Degrees Celsius, which means it can reach the necessary temperature for "browning" to occur.
Can I sear a steak in water at 10 atmospheres of pressure?
Interesting question.
It seems that the short answer is yes and no, there is some Maillard reaction that goes on at high pressure (400 MPa or more; normal air pressure is 0.1 MPa). However, this has only been studied in model systems using glucose and lysine in buffered reactions, so by default this is in a liquid system, not a solid-liquid system like you describe. It seems this is also largely studied at fairly low temperatures, around 60 C or 140 F.
I found a fairly recent (2003) study here, with the full article here (likely paywalled) that indicates there is a strong pH dependency to this. At pH 7 (neutral) there is an inhibition from pressure, but as you become more alkaline (up to pH 10) there is some promotion of Maillard reaction, even at standard air pressure. A similar study (also paywalled probably) in 1991 found similar results up to 500 MPa.
The study also looked at browning and found a strong inhibition of browning by pressure, with almost no browning of the solutions taking place at high pressure.
However, in your system you are only looking at 1 MPa, not the very high pressures described in the articles. There is not really any precise way to extrapolate from the data in the papers to your system (liquid-liquid vs solid-liquid), but it looks like you could undergo the Maillard reaction, and probably brown too, but I can't say for sure.
