I've thought about this with my Press Graflex.
It would involve making a switch that would close when the focal plane shutter was open, say by putting a foil patch on the edge of the shutter curtain and a couple of contacts that would close the circuit when the shutter was fully open, but lacking an instant return mirror, you would still have to close the mirror manually fairly quickly to avoid an excess of ambient exposure--not much better than normal open flash technique.
A really effective but more complex way to do this would be to use the same type of focal plane shutter switch, but have it actuate a solenoid that would fire a leaf shutter on the lens, that would sync to flash normally, but then you need a lens in shutter with a solenoid that all fits in the Graflex, and you lose the option of using barrel lenses.