On concertina.info Chris Timson talks about an anglo he has which has been converted to a MIDI instrument by Roy Whiteley of Accordian Magic, and which he plays via a synthesiser. The concertina has pressure sensors which detect the direction in which the bellows are moving, but Chris's article doesn't say whether these also change volume. As it is a vintage instrument(and irreparable when Chris got it)it has the added advantage of being an attractive thing to see, rather than an oddly-shaped piece of plastic.