I did it: I send 'external' data via the Nabaztag to the internetz. For that I replaced the RFID reader with an Arduino, running a program which simulates the original communication completely, however, allows to send any data to the rabbit! Yay
Unfortunately, with the default firmware, the bunny reads only about 8 bytes / s, but with my own boot code cloud get far higher data rates.
Take a look: Readme, source code, etc. are on blog here:
Looking forward to your feedback - Tobi