It would probably work a bit. But what most people tend not to know is that the Maillard reaction works better at high pH, that is a base environment, ph > 7. If you brew beer well, then the pH of your wort will be around 5.5 cold, and 5.2 hot, and in a pressure cooker, the temperature will be higher, and so your pH will again be lower. So it will not work as well as you would think, except for the temperature of course. I have used my pressure cooker to sterilise around 7 ltr of wort, to then let it cool overnight. Since Maillard reactions add color, I should have noticed that afterwards.
And about decoction for promoting melanoidin production: no, it isn't. Decoction is an old technique to make a multi-step mash, using some simple rules about volume of hot (boiling) mash to increase the temperature of the mash in steps. Originally, the first step at 35° C was to increase the acidity of the mash (well, people didn't know that back then, but someone must have found out that it provided better beer), and more acidity is less Maillard reactions.
The only step which might have been done to improve the color (and possible the taste) was the last, were thin wort was taken, brought to the boil and boiled until the color was slightly darker, and which was then used to mash out.
I have brewed a couple of (blond) beers using a decoction, starting from 45°, to 55°, to 65°. They were still blond. Did they taste better? Difficult to say. I brewed the same beers without decoction, and they were equally well.