Looking at the picture above I'm thinking that the stiff firewire cable is making the external battery solution a lot messier than it needs to be. I don't like spaghetti, and I think the sync cable to the shutter is bad enough.
What I'd like to have is a tight L-firewire plug and then an easily separated contact on the middle of a soft cable to the battery, so when you accidentally pull the cable it'll split rather than pull in the firewire input in the back.
I had been thinking that nhe battery is small enough to make a pocket and tape it to the free space on my Linhof sliding back too, so you would have left-to-right: digital back, ground glass, battery. But I just realized that that doesn't work as the sliding part is only two sections of course...
you could tape the battery to the opposite side still but then the cable would not be of constant length. I'd very much like to avoid having an extra cable to attach when I set up the system (in fleeing light one needs to work fast!).
A third alternative would be to have the battery in your jacket pocket (great in the winter!) and have such a split-cable for safety. I don't think there's split cables available though, one would have to make one.
I don't think it's a good idea to always insert/remove the cable from the back, it would be unnecessary wear to the digital back firewire contact. Better to have a short cable permanently attached with a easily slide-apart plug which you then attach to the camera battery cable.
Of course there's the CFV-50 too if one can get it, then this external battery mess is avoided. However the H4D-50 has a little bit better screen. After seeing photos of it compared to the H3DII-50 screen (which I believe is the same as CFV-50) I'm not so sure that the H4D-50 is significantly better and not sure that the H3DII-50/CFV-50 screen really is unusable for focus check. I'm a master of interpreting bad screens, so maybe I can make sense out of it. It would be great to test, but buying seems to be the only way to test in my part of the world.
It seems to me that the key problem of the Hasselblad screens is that pixels are not square, there's some special type of subpixel arrangement that make the pixels look fuzzy and thus it's harder to interpret the 100% view.
In this post at Lula you can see a photo of the H3D-II 50 vs H4D-50 screen:
http://www.luminous-landscape.com/forum/index.php?topic=93577.msg763349#msg763349
It's hard to figure out from a picture how it looks in reality though. On the H4D-50 screen you do see jaggies in the grass, and jaggies = sharp, so that indicates that you can see if something is sharp. I'm not sure from that image if the H3DII 50 screen makes those jaggies visible.
If one gets a H4D-50 for the sake of the screen one need to make sure that one gets a "real" H4D-50, there are also versions that are upgraded H3DII-50 and those have the old screen. One also needs a late firmware for the H4D-50 to enable the higher resolution.
Unfortunately I use 100% view extensively on my Aptus 75 today so I'm not so sure I can live without it. It's not so much for checking that I hit focus but it's for things like checking if the background is "suitably out of focus", or checking for suitable focus compromise in complicated tilt/swing closeups (where Alpa/Arca style HPF focus ring would not work either). Sometimes in really difficult light conditions I'm also lazy and guess-focus and adjust after looking at 100% view, with discipline I could solve that otherwise but it is indeed nice to have the option to check sharpness with good precision. The Aptus 75 has certainly not a great screen either, but pixels are square and the demosaicer is sharp(ish) so you can differ between super-sharp and very-slightly-out-of-focus, not with as good precision as you can when pixel peeping on a good computer screen, but good enough to be very usable in the field.
I guess that you that are used to shooting film should have less desire of having a working sharpness check on the back?
