I finally managed to upload the Micronucleus Bootloader and getting it to work.
I am now able to upload sketches via USB (after resetting the chip manually with the push button).
I may also could now upload sketches using the Arduino IDE, but I like to use GNU make instead.
I uploaded a simple blinking sketch using the on board LED on PB2. For a more anticipated test I didn't have enough time, since I wanted to upload this Log as a proof of concept for the square inch contest.
look ma, no hands: