Hi LMNOP. I am in a very similar situation, wanting to have a single flash system for both my Canon EOS and Mamiya AFD systems.
The only option as has been said above, is the Metz MZ series using a different SCA adapter for each brand of cameras: (in my case, SCA3102 for Canon EOS digital, or SCA3952 for Mamiya digital).
Any MZ-series flashgun will work with any brand of cameras. 40MZ, 50MZ, 54MZ, 70MZ and 75MZ are the ones I know of. The Hammerheads (50, 70 and 75) are definitely compatible with both SCA3000 and SCA300 adapters. The other two (40 and 54), I do not know.
To figure out which adapter you need, the index numbers on the SCA3000 translate as follows:
SCA Nxxx is the name of the entire sytem. They are all 3's.
SCA xNxx is the name of the brand it will TTL for: for example 1. Canon. 4. Nikon 9. Mamiya
SCA xxNx I dunno.
SCA xxxN film or digital. 1 is for film systems. 2 is for digital systems. I think digital will be backwards compatible. It should?
e.g. SCA 3101 is for Canon EOS film cameras, while 3102 is for Canon EOS digital cameras.
e.g. SCA 3951 is for Mamiya AF (no digital back support), SCA 3952 is for Mamiya AFD and up (digital back support).
The SCA3000 system does not cover the much older 35mm film SLR's such as the Canon A-series.
I think the SCA300 adapters will have one for those. I'm not sure if these will meter TTL but you can look it up.
Hope this helps. I am still looking for adapters for my systems.