Actually just took a quick look at this, and it seems that USB MIDI isn't a HID device after all (at least looking at this code), which means a report descriptor may not be enough.
If that's the case I'm afraid you might be out of luck - it'd require changes to Espruino to let you specify the whole config descriptor. I hadn't done that because it's actually very device specific (setting endpoints and suchlike) and it also contains stuff to do with the USB com port as well.
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Actually just took a quick look at this, and it seems that USB MIDI isn't a HID device after all (at least looking at this code), which means a report descriptor may not be enough.
If that's the case I'm afraid you might be out of luck - it'd require changes to Espruino to let you specify the whole config descriptor. I hadn't done that because it's actually very device specific (setting endpoints and suchlike) and it also contains stuff to do with the USB com port as well.