If you've held the play button for a minute and your iPod has not powered off, then something is broken. It shouldn't take nearly that long, maybe 5 seconds at the most.
It doesn't matter which screen you're in in Rockbox - this works from anywhere.
It's also not the same key press as "Long Play" - There are actually three seperate button push events on the Play key, "Short press on Play", "Long press on Play" and then "Extremely long press on Play". As JdGordon says this is simply due to a lack of buttons on the iPod.
The only way to map shutdown to something else on your iPod is to change the source code and compile your own build.