Well my friends at aphroditestore have turned up with the goods, and have delivered their tiny Leonardo-compatible board. It's only about 2cm x 3.5cm, and has a nice 32U4 processor on it, a micro-USB and enough I/O for the hakkedpad. It's also small enough to hide inside the case, so no messy dangly bits ..
The designers of the Launchpad thoughtfully left some diamond-shaped pads to make it easy to solder to ..
This made it easy to wire in the 'Pro Micro' Leonardo board .. I had to change the arduino code a bit, since the ports don't quite line up with the Leonardo, but the arcore extension loaded fine, Renoise spotted I had added a USB MIDI device ..
Time to stick the tiny board onto a couple of sponge pads, and wire in a short micro USB cable to the existing USB connector on the Launchpad - that way, the 5v serves the existing logic on the motherboard, and powers the micro board as well. It also means I can avoid trying to solder anything onto the micro USB connector which are too damnably small to see. Even with the surgical loupes.
And, ta-da! Hakkedpad in action ..