If only the flash did not pretend to have to make love with the camera and just bloody worked when the camera tells it to work, everything would work because, as far as I know, using synchronization for bulb flashes (M, FP, whatever) always makes the camera fire the flash before the first curtain begins its travel, because bulbs need a small time to ignite so they have to be fired before the camera.
It's a design "flaw" of those new "FP" flashes. They want to set themselves the camera to FP synchronization rather than X synchronization, and if the camera doesn't support FP synchronization (or doesn't have a way to tell it that it does, as is the case with old cameras) the flash will only work with the normal X synchronization method (one flash). It's also quite possible that those dedicated cameras cannot be set on FP synchronization on their own, only the flash can do it, so that one has to buy a flash of the same maker.
If only there was a way to tell the flash "trust me a long emission is what I want, and I know better" any of those flashes would emulate a bulb flash and be compatible with any shutter speed on any camera with a bulb flash synchronization.