Unfortunately, the accepted answer is off the mark. :-)
Espresso is extracted at 9 bars, even in so called "good" or "decent" espresso machines. The source of this misunderstanding is that most manufacturers state the rated pressure the pump inside the machine is capable of delivering (and consumers are often led to believe that bigger is better). The pressure is regulated to 9 bars when the espresso is actually brewed (with slight differences possible depending on the individual service setting of the machine in question). There is a pressure relief valve that simply sends the excess back to the water tank. Espresso coffee extraction is an equilibrium of many factors, including quantity, temperature and time. It's actually not even desirable to have a higher pressure than the traditional, tried and tested 9 bars, together with all the other standard parameters used.
What's more important in the context of the question is that an espresso machine is not a pressurized container of hot water. The 9 bars are created by a pump that only operates while the machine is actually pushing the water through the ground coffee. The amount of water is rather small (a serving of espresso is 30 ml, it's usual to have two-cup sprouts, meaning 60 ml). In any event of breach of pressure (eg. the portafilter accidentally knocked off), the pressure will fall immediately because it is only built up against the ground and packed coffee, nothing else inside the machine. With no resistance, the pump simply continues to deliver hot water, like a tap. So, the result could be a small amount of scalding hot water for anybody standing close, which is a nasty thing but certainly no risk to general aircraft safety.
Actually, it could be added that using a boiler to keep any amount of hot water is more characteristic of the traditional espresso machines used in catering and by connoisseurs at home. The kind of machine more likely to be used on board of an aircraft, the so-called superautomatic type (this is the combined machine that grinds the beans and brews the espresso at the touch of a button) tend to use heaters that simply heat the water flowing through them during the brewing process. This reduces the startup time of the machine and requires less electricity, although it produces espresso of lesser quality.
Because of this setup, the pump would deliver the same pressure at any flying altitude. It's the pressure relief valve that actually decides the pressure, not the pump. If it stays set at 9 bars, that's what will be used during the brewing process. What the altitude would influence is the temperature we can heat the water to before it starts to boil.
Espresso brewing needs to avoid boiling water at the usual ground level primarily because of the temperature limit of 100°C. It's not the boiling itself that would ruin the espresso (apart from the fact that steam, being compressible, would not work in a pump set up to send high pressure water through the coffee puck) but the high temperature: the aromatic ingredients in the coffee bean can only be extracted, exactly as the OP has stated, in a rather narrow temperature range and going near or above 100°C simply ruins the taste. In this respect, if ambient pressure differences would decrease the usable temperature range at higher altitudes (say, 92°C being the highest temperature of water that can be achieved before boiling), this would only require a different selection of coffee on board, one that tastes good when extracted at that temperature (probably meaning that both the composition of the blend and the roasting process should be adjusted accordingly but this task would be simply left to the supplier).