I figured out how to restart nickel without rebooting the Touch, so Tetris can now
be started from the Kobo's home menu using ah-'s plugin interface!
The plugin, with an updated version of Tetris, can be downloaded here.
Extract the file and copy KoboRoot.tgz to the .kobo directory on your Touch.
You must open the library once to start the plugin.
Pressing the home button or pulling the power switch will end the Tetris game
and send you back to the Touch's home screen.
This is HIGHLY experimental, and things might not always work right!