A while ago I contacted the author of RTMIDI to find out about possible JACK MIDI support. He said he'd accept patches to add JACK MIDI support. I was going to try my hand at it, but have gotten busy with other stuff and may not have much coding time. So I'm passing this on in case anyone wants to have at it.
RTMIDI is a multiplatform wrapper around various MIDI transport methods (Core Audio, ALSA, etc.) and is used by several different programs and language environments, particularly by ChucK. Since ChucK is a strongly-timed language on a sample-accurate basis, it would probably be useful to have it support JACK MIDI's sample-accurate MIDI timing too.