Jack on MacOSX:
There is Jack for MacOSX but there is also ReWire for MacOSX. Renoise side-host hooking is based on ReWire on Mac and Windows so you won’t get Jack linked from within Renoise on Mac (Just as you won’t be able to do this on Windows either as there is also a compiled version of Jack on Windows.)
There is however also a discussion going on to switch to jack on Windows and Mac as well due to the fact that ReWire doesn’t support 64-bit and there aren’t plans to update the ReWire structure for 64-bit.
Jack Midi capabilities:
I didn’t had problems routing midi in and out of Renoise using Jack on Linux. Limitations are however on the plugin area and Renoise doesn’t support internal midi routing between plugins yet (on any platform)