The vol+ button is just a regular old gpio. Treat it as such and you can do whatever you like with it.
The power button is a special signal that feeds the PMIC. I think there are different ways to handle a power button “event”, but I think it will always be just that. Try HOLDING the power button down. I think it will eventually do something. Also watch the kmsg when you press it, if you’re lucky, it might say something to you. This signal is also attached to pin 4 of the low speed expansion header. See page 15 and 17 of the hardware manual. Note that the manual describes this button as being able to trigger wake/sleep states, but NOT power the board off. This is mainly because the board doesn’t actually have an “off” mode.
The vol- button is another that is tied into the PMIC. Odd thing about this one is that it is actually the RESET signal of the PMIC, which has been dual purposed to serve as vol- for presses lasting under 10 seconds. Holding it for 10+ seconds will cause the board to reset. This signal is also connected to pin 6 of the low speed expansion header.
So… you can do whatever you like with the vol+ button, but the other two are pretty much set in stone.