Investigating with Wireshark on Linux + USB Passthrough on a Windows VM. There is a transit of what seems to be "regular" MIDI inputs.
After searching a bit, it seems to be full MIDI \o/ : https://support.roli.com/article/controlling-your-instrument-with-the-seaboard/
I'm using Google Chrome's MIDI API to make a web hack for the roli, Upon connection opening (MIDIPort.open()), I successfully get MIDI messages.
I get "Note ON" and "Note OFF" messages. Which allows me to implement the first touch detection.