Venusian Snail Traffic (Mokafix) has released Harmoniac, a physical model reed which can produce harmonica sounds.
Harmoniac comes as a VSTi plug-in which works as a synth, and as a VST effect plug-in which has MIDI In to play notes. In this way it can be fed audio to get a natural blowing sound.
Visit Venusian Snail Traffic for links to download Harmoniac VST and VSTi. Sound demos are available as well.