Nice work! You're even lazier than I am.
I have a foot pedal set up like that using a scrounged keyboard.
I use the PAGE_DOWN codes (which wasn't formalized until after our Android 2.1).
I didn't bother tracing out anything, besides, I only had the tiny PCB itself.
I just ran a shorting wire over stuff until I saw the right keycode on UsbMode.
You may have noticed already, we need to do some work on getting USB host mode to use less current.
Right now, it uses a lot, between 50 mA and 150 mA.
That's a lot, considering that the idle current is less than 10 mA.