How to use jack midi with nightly?

I'm trying to enable jack midi output, but I can't figure it out.

I'm using the latest available 64bit nightly, rev. 789ee8b, from http://prereleases.musescore.org/linux/nightly/ and ubuntu 13.10.

My I/O settings are : enable jack midi out, 2 ports,
enable internal synth, jack audio output.

Anything else selected in internal synth, and playback doesn't work while jack is running.
Disabling the internal synth doesn't actually work, it's enabled again when the program is restarted.
If I disable it from the command line with -s, the play button is greyed out.

I see a mscore midi port in qjackctl, but it produces no output with these settings. I'm connecting it to gmidimonitor which should print out anything coming in, and which works with other ports.

There's a somewhat recent issue, but it has no comments or other info : http://musescore.org/en/node/22814