Momentary/Pushbutton Power Switch For Handheld?

[obsidianspider]

I know that Mausberry can handle a momentary switch for toggling power, but that's only for something that is plugged in all the time. I'd like to know if anyone has figured out a good way to do it for a (battery powered) handheld. Long down my list of future builds are a Wii U controller and also a WonderSwan, similar to what @goosewerks built, but the WonderSwan Color used a button for power, instead of a SPST switch.