Not sure exactly how the PW-70A + ITPS works, as I use an OPUS, but I'd imagine the power supply is hard wired to the battery, and recieves 12v+ all the time, even with the ignition and computer off?
If so, then it should be just a matter of hooking up a momentary switch to the two power-on switch pins on the motherboard to manually turn it on/off when you'd like.
Another route would be, if you use a wire hooked up to your ACC line (or something like that, a power source that only supplys 12V+ when the ignition is on), is to wire a switch to that ACC line that goes to your computer, that would give it 12v+ whenever you flip the switch, even when the ignition is off. Sorry if that doesn't make sense, I don't really know how to explain what I mean. Perhaps someone else can make sense of my gibberish.
(Yes, I'm in terrible need of sleep. )