Hi Toby,
I've been recording melodeon over several years now, including the great English folk melodeon player, Tony Hall. Having gone through every different mic placement option (many recommended by the seemingly great and good), I found that instead of any stereo pair in front (which all sounded dismal) the best way to mic was as Tony himself suggested, but which I doubted would work. That is mics aimed pretty much at each other (slightly angled towards the player), either side of the instrument, set back enough (not least for the movement of the left/bass hand not to result in uneven volume), and not panned hard left and right: you'll need to play with that. I think this general placement is more crucial than the mic model or, even, polar pattern: on which note, I found SDC omni mics worked best, but I have been recording in a reasonable (medieval) church acoustic.
The photo should give you an idea of the variety of options I was using.
Hope this isn't a red herring for you: certainly give it a go.
Cheers,
Roland