Hmmm, just a couple thoughts off the top of my head...
The Boss octave box (or pitch shifter, for that matter) fits in a pedal, so the electronics required to do this are probably not that large. It does suck batteries, though, so I would make sure this was running on it's own power source. You would want the off setting to be a complete bypass in the event of battery death. After adding the switch and the battery, how much room do you think you have left for a circuit board to do the work?
You would probably want at least two pots for this in addition to the switch, though they could probably be mounted internally and controlled by mini screwdriver. These would let you balance the relative gain of the two octave effect signals against the original signal. I wouldn't want it hard-wired at any particular level.
I think Korg used to make a box that did both an octave up and an octave down, but most of these things either just do lower octaves, or are variable pitch-shifters. I think the electronics will have to be custom built to get what you want rather than just pulling the guts out of a pedal and slapping them in your guitar. I hope that they can design a circuit that has the sound and quality appropriate to the Alembic name if you're going to stick it in one of their basses.
Sounds like fun!
