1) I'm sure it was unintentional, but the "hate to break it to you" bit comes off as very arrogant and as if you're automatically assuming the person you're talking to is either 5 or an idiot....
2) I personally have not ever actually put a pressure on a cooling system under load to watch what the pressure does, but I suspect it's very unlikely that it stays constant unless it stays at exactly the same pressure constantly. The radiator cap is not really a pressure regulator - it's basically just a simple pressure/vacuum relief valve. That is, when pressure in the system exceeds the set (13 psi) it will release pressure, and when the system pressure drops below atmospheric (ie when the engine cools off) it opens allowing the system to refill from the overflow bottle. However, when the pressure in the system is between those 2 limits, the cap does nothing.
So, think about what happens when you're pulling a hill.... the system is going to heat up to say 220* causing a certain amount of expansion, pressure and flow into the overflow.... then you get to the top, load drops and engine cools back down to ~180*, with a consequent contraction of the coolant - pressure will now go down from what is was at 220* and this will become the new system pressure until it either cool off enough to pull water back in or heats up enough to hit or exceed 13 psi again.....
There's also a degree of surge in the system due to rpm change - try running the truck at high rpm with the cap off and you'll notice that the radiator level changes a bit. Not sure if this would be enough to matter for a pressure sensor though. Depends too on where you put the sensor - if it's on the outlet side of the water pump there will certainly be a pressure change with rpm.
I have personally had experience with a slow leak that caused a loss of water but would maintain pressure - I think it was caused by head cracks and would very gradually loose coolant but would fill with air/steam and maintain pressure. You could also have a leaky overflow hose or empty overflow tank which would cause the cooling system to develop an air bubble in the top tank. I don't think this would be big enough to cause an issue, but it might be. In either case, the pressure sensor would show pressure, where a level sensor would tell you the level was low. The problem I had would get bad enough to start trying to overheat though it never got bad enough to really be bad. The coolant level sensor would catch it well before it ever got low enough to start overheating though.
The level sensor setup I have was very simple to install - it goes in the top tank. Just drill a small hole (~3/8" IIRC) and solder in the bung.
You could also just use a float sensor which would be cheaper and easier to wire, though more difficult to install. You might have more issues with slosh though. The one I have is capacitive - no moving parts.
I will also note that I have never heard of a coolant pressure sensor being used anywhere for an alarm system, whereas level sensors are very common both for industrial and automotive OEM use.