I received an email from Jordan this evening. The MBE can be configured to play the same note on the press and the draw. This will be great for building an English or Hayden (or other duet) concertina.
Judging from the description, the MBE looks like a nice all-in-one solution for the MIDI concertina. My own home-brewed MIDI circuit is pretty much the same thing, only not so tidy. I have a PIC microcontroller polling a pair of pressure sensors (in the MBE picture, the pressure sensor is the oblong black hunk in the upper left corner) and scanning banks of buttons.
Here's a warning for you: this MBE gadget (and many others, including my own project) are designed on the assumption that your buttons are just electrical connections that are either open or closed---standard pushbutton switches, for example. You might instead opt for hall effect switches---these are sensors placed under the buttons or pads, which detect the presence of a magnet connected to the button. These have the advantage of being mechanically reliable (no contacts that may or may not contact, and no bounce,) and allowing you to make your own buttons.
The problem is that most hall-effect switches have TTL outputs, meaning that instead of being a connection that's either open or closed, the switch outputs a voltage that is either high or low. This is actually quite convenient if you design for it, but some effort is needed to interface a TTL output to a device what expects to see a closed/open switch. Your MBE will have multiple banks of buttons connected along the same wires; bad things can happen if they are not correctly isolated from one another. Bad, burny things.
In the end you can wing this with a diode or transistor on each sensor, but no matter what it will be more parts and effort than pushbuttons, which can just be plugged right in with no fuss or muss.