I think that's the wrong way round. I think there should be a launcher in the flash that starts instantly when you flick the power switch and lets you launch either games/emulators from the SD card or boot Linux.
Errm, I know it's not the best solution, but if you go into the musicplayer and press Select, it switches the LCD and backlight off
Not much I know, but it saves me a couple hours of battery-life when I use it for playing MP3s
There isn't a way yet, but I'm sure it wouldn't be too hard to do. There's already an clock speed/screen refresh option, and I bet the devs also know how to turn off the backlight. It would probably take a good one a few minutes to make a simple "sleep mode" app.
An actual BUTTON is HIGHLY unlikely. But as for a button combination or assigning that function to a normal button while in the menu, you can always ask RiX0R to implement it in Mocca if you plan on using Mocca for your menu.