I think you've just summed up the problem with almost everything about the internet these days, in that the information doesn't exist unless some nutter/collector goes and puts in on the net (and the older and less popular it is, the less likely).
Deciding on my first DSLR I ended up going for Canon, not least because of friends' recommendations, but also from the massive wealth of information provided by TDP, the canon camera museum is also great for all their FD and LTM lenses and bodies.
Same for when I switched to MF Film, the best site that I found was PentaconSix.com, almost everything written about everything, just maintained (still) by one guy who just loves them (and has the funds). Just having that information available meant that my first MF was P6, now I've got almost all the zeiss and soviet lenses, and a few soviet bodies to boot. They may not be the best ever made, but at least I knew what I was getting myself into.
I've never found a 'definitive' equivalent for Nikons (although KR comes close, and I do find his Leica pages invaluable, even if not definitive), and there's dribs and drabs on P67s, a nice site on Rolleis, I've never seen anything much written about Bronicas at all.
For mamiya 645, same as you, I've gotten all of my info from wikipedia, camerapedia, and just seeing what's out there and popular on ebay and keh.
At least the leafasemiya website does have old user manuals for bodies (but I'm not sure how far back they go though), so at least you're getting more 'official' info than most systems...