There doesn't seem to be any threads in this forum or anything specific on this subject when I search the rest of the internet.
I would open up my camera but it was recently repaired and working perfectly. I might get a camera for parts on ebay, fix it and then try to rig this up.
I don't like optical slave flashes. There must be a simple signal for the internal flash that can be tapped and used to fire the flash in the hot shoe. At most it should only require a diode, scr, opto-isolator or maybe just some wires. There shouldn't be any reason why a two or three way switch couldn't allow for selecting internal, external or both flashes to operate in sync with the shutter.
Has anyone given this a try yet?