Hmm... never actually hacked that one, I don't know if it is even hackable.
I always use official and MadCatz pads for Dreamcast
The Mad Catz pad is the way to go, though I can't bring myself to hack one, due to them being about the best pad for the system, in my opinion. It's probably that pad's fault that I favor the Duke on Xbox over the Controller S.
Most everything I've read about the Quantum is that the extra face buttons mimic two existing face buttons, leaving them hackable but useless as far as the extra two buttons go. However, I've seen mention of them being programmable in the same manner as the Mad Catz pad. Not sure, really.
I've hacked my Agetec stick, so I'll just be using the existing board as a separate encoder.