Just thought to add to this thread in case someone is wondering about the same thing.
So yes, the Nikon External Battery Pack designed for the old F-series of Nikon SLRs (e.g., FE, FM) does work on a Mamiya 6. You just need to install the two AA batteries in reverse of what's indicated on the pack. The reversal compensates for the fact that the way button batteries are installed on the Mamiya 6 is opposite to the way the button batteries are installed on the F-series SLR bodies. The Nikon catalog number for the external battery pack is DB-2.
