These days, the world of 'larger' sensor photography just boils down to either Phase or Hasselblad, but there are some interesting variations within the Phase 'family'. I have been using the Mamiya 645 AFDII with the ZD back for the last two years. Generally, people seem to reach for the spitoon whenever the ZD is mentioned, but as I understand it, it uses a 46x38mm 22MP sensor and the exposures will go out as long as you like (they are horribly noisy though beyond a minute). Mamiya still make adapters to put this back onto the RZ67 ProII SD and onto 4x5 cameras. The performance of this back is poor at the extremes of dynamic range, but make no mistake, you can get good images out of it and it has an interesting (and I think unique) trick up its sleeve in that it can do true infra-red. It is just a shame that corporate priorities are likely to have dictated a lack of support for this product in terms of firmware support, because it is cheap, it is self-contained and it is practical (takes SD and CF cards).
I am just in the process of buying the Leaf Aptus 75S (will probably keep the ZD as a back up and to do the IR). I am told this is essentially the same as the Aptus II 7, but with a slightly dimmer LCD screen. Reviews on this site and elsewhere seem to indicate that this back is competitive with the P45+ and I think I am right in saying that it does not suffer with the colour shift issue when using movements. It is less self contained than the Phase back in that the battery hangs off the bottom of it, but that is the price you pay for the large LCD screen. I was told that the Dalsa based sensors have a more 'filmic' quality to them than the Kodak based sensors (as in most of the Phase backs) and I am certainly very impressed with the quality of the images out of this back. That really just leaves the issue of the longer than 30 seconds exposure thing (which it cannot do). Personally, I cannot see what the problem might be here. You get 12 stops of DR out of the Leaf back. Unless you want to shoot in really low light (and I would use a current full-frame DSLR for that), it should easily meet your needs.
As for the LF thing, well I have spent the last 25 years shooting LF to get the photographic quality and speaking personally again, I think it is a massively overrated pastime. I cannot tell you how much of a relief it was to finally get rid of all my LF gear finally about eighteen months ago. However, I have been thinking about an LF type camera to go with the leaf back, if only so that I can use say a 24mm SA XL. I would forget about your Canham or just about any 4x5 camera and I would want a totally rigid setup with the lens in a helical mount. Depth of focus issues could be dealt with with say Helicon Focus and I might be interested in a bit of rise, but even then, not that much. I would look at something like the Alpa TC (light, cheap and totally rigid) or the Cambo Wide RS with the Schneider 24mm lens (Calumet seem to be doing a good deal on that just now).
Just my slightly alternative thoughts on your conundrum...... Oh and no one is going to give me a spanking for saying the 'wrong' thing(!)
