I know this is a bit off topic, since the OP is specifically asking about the Mamiya 6 which has a leaf shutter. To that specific request, I cannot give a reasonable answer. Perhaps the Pocket Wizard is the best solution, but I'd call and ask if the delay mechanism on the device is suitable for what you'd want to do before spending ~$200 for the thing.
For any other folks interested in doing something like this on the real cheap, I can offer some help. Rear curtain sync works with my very old and battered Nikon N90s, and it will work with ANY flash unit. I use it with an equally battered Vivtar 283 and the rear sync works like a charm. Set the body to trigger the sync in rear curtain mode and you are done. These bodies can be had on the used market for very little money. If you already have Nikon mount lenses, you are good to go.
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Frank Schifano
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