Hi Lars,
I have the same problem and from what I understand, the problem is that the alsa driver for the rpi lacks functionality ( mmap support) required by jack.
I was thinking about writing a new jackd driver backend using the omxlayer available on the rpi, and bypassing alsa, but before doing that I would need to know more about the jackd driver interface and what functions would be required to implement. I am only interested in audio output, so my aim was to limit the driver to the required output functions.
Any help, suggestions or guidance would be most welcome.