Do you think that you could witch the JUCE code for Windows to DirectMusic, i think this could help with the multi-client sharing of MIDI devices, a lot of programs do that and it helps if you want to open the device more then once. I’m getting some bug reports from people trying to open devices that should be multi-client but JUCE crashes when trying to open such devices.
I was wondering have you considered switching to that API ?
There is a nice overview and a demo here http://www.codeproject.com/KB/audio-video/midiports.aspx
and a opensource DirectMIDI implementation (i guess a library over the SDK) that helps with this http://directmidi.sourceforge.net/