A member of the Haiku community has made an early prototype port of JACK2 to the fledgling, media-centric FLOSS OS

https://www.haiku-os.org/blog/barrett/2013-07-02_jack2_personal_analysis_part_1

TL;DR

Haiku currently lacks RT scheduling and shm.h and this proto port lacks a backend to produce any output. The author proposes it may be possible to create a bridge similar to JackRouter on OSX to pipe JACK to Haiku's media_kit.