A rectangular panel of wood or perspex resting on top of the harmonium, over the reed bank. Absorbent material between the surfaces to minimise mechanical noise and prevent slippage.
Two holes in the panel to accommodate 2x Silver Bullet mics (I'd fix them with silicone sealant) are connected by XLR to a 2 channel mini mixer (can also be fixed to the panel). Thence to the PA.
All in one unit, just one power connection.
An element of acoustic isolation will reduce feedback, high end at least.
Headphone output on mini mixer can be used for monitor.
Relatively cheap (I reckon less than £60 for the tech bits).
Might not be the ideal distance for the mics.
Not the most elegant solution.
I may be completely mad.
PZM on the panel, no mixer, otherwise as above. PZM can be phantom powered.