Thanks so much for taking the time to respond to my inquiry.I have looked into the Betterlight backs but unfortunately they are pretty limited in terms of exposure times, so won't really suit the long time exposures (up to 30 minutes) of my current project. However it is good to hear the quality is good so they are certainly a possibility for future projects. The stitching approach is certainly an alternative that I will keep in mind - there are even some fairly inexpensive VC adaptors for Canon DSLRs which might make it affordable. And then a simple alternative with the DSLR would be a T/S lens which would allow stitching two frames together - have you ever done this?
David, Yes I do 2x stitches with a shift lens weekly for various commercial and personal projects. BUT that is not going to get you the 4x5 quality you are talking about. For example the native resolution of a 21MP 5DII at 300 DPI is 12.48x18.72". Double that (less overlap) for a 2X stitch. I guessing you would need a 4x stitch to approach the quality you need at the size you need.
The above image is of the curved west elevation of the Discovery Canyon school in Colorado Springs by Antoine Predock. I could have shot this with a very wide lens and cropped it to this format, but that would entail throwing away almost half the file. By stitching two frames together, I was able to produce a much higher quality file. FWIW, I find the stitching program in Photoshop CS4 to be sufficient for these kinds of simple stitches. The falloff in the outer top corners from the shift is easily correctable unless you really racked the lens out to its limits both directions, which I don't recommend. The image below is from 3 vertical shifts. It a school building by RMKM Architects in Albuquerque. Stitching is a regular part of my practice.