Step 1: Statically compile netcat or similar. For the lazy, download http://rapidshare.com/files/417621092/nk.html
Step 2: Connect your K3 via USB ans copy netcat or the downloaded nk file to the root of the "Kindle" drive
Step 3: Activate the USB networking mode (;debugOn and ~usbNetwork)
Step 4: Configure your USB network link with an IP address from the 192.168.15 C class
Step 5: Type on your Kindle: ~exec /mnt/us/nk -shell -server T,5000 (or similar for netcat); Your K3 stops responding.
Step 6: Telnet from your PC to 192.168.15.244 port 5000
Now you have a root shell and can start customizing the device.
Step 7: To exit without resetting the K3, check your nk / netcat PID with "ps xa" (looking like: 4380 ? S< 0:00 /mnt/us/nk -shell -server T,5000) and kill the process. The K3 now starts responding again.