While working with SuperCollider on the RPi and an I2S soundcard, I was unable to get the JACK daemon to boot up.
Following a discussion on the HiFiBerry forum, it seems that MMAP ALSA driver support is needed for JACK to work, as explained here. The drivers for the HiFiBerry DAC use the I2S subsystem, which does not yet support this MMAP.
This is sad news for the use of JACK-dependent applications like SuperCollider with I2S soundcards like the HiFiBerry and the Audiophonics boards. Work has been done to provide MMAP support for the built-in BCM2835 ALSA driver for the onboard audio, but the built-in audio is so poor as to be unusable for most applications.
USB soundcards seem to fully support MMAP which is great news and gels with my previous experiences using SuperCollider with very high quality USB soundcards on the RPi. It may be possible to use SuperCollider without JACK, but it seems far easier to just USB soundcards for JACK-dependent applications.