Well, the flash must support second or rear curtain sync too, or it isn't there to use.
And that is the thing I really don't understand. What great magic must the flash know in order to be able to rear-synch? I mean the camera sends the signal to fire the flash before the shutter closes and that's all the flash needs to do. This is especially true for bulb mode since in that case Canon cannot even do it's famed 'I'll send the shutter speed setting to the flash
' since the camera doesn't know the shutter length anyway. So it should just send a dumb 'fire now' command.
I have done the combining of flashes that you described but that is pretty much useless for me. I need all my flashes to fire on the rear curtain and not have some of them fire on first curtain and some on the rear curtain. (Well, I guess I could see one or two applications were this would be a fun thing to do, but not for the majority of my pictures)
This is turning out to be much more annoying and certainly a lot more expensive than I thought it would