Wow. I've been a Phase One partner for 16 years and have no recollection of there being a "CB68" – Wikipedia says it was one of the absolute first products – 1993. Because of the mechanical parts (the moving scanner-like mechanism) most of the Phase One PowerPhase scan backs I've come across are not in good condition (not working or produce significant streaking or banding) and those are from 1996 to ~2000. I suspect they were more reliable than the CB68 (only a guess of course; I have no empirical data, but generally companies making iterations of a core product improve it over time as they see what tends to fail in the previous model and better learn to assemble and test that product). So finding a working CB68 is a long shot I think (as you said).
Software would definitely be a limiting factor as well. Phase One maintains accessibility (via support; not the website) to more of their legacy software than most companies, but 1993 is definitely pushing it. This back would have zero functionality until connected to a computer running the required software. I assume SCSI connection? Though thankfully it's still feasible (though challenging) to get the appropriate computer hardware, getting/installing/configuring the software would be a journey of its own.
None of this is to dissuade you – it's a neat project, and I wish you luck in it.
Let us know if you succeed!
If you're not specifically "stuck" on the 680III then a PowerPhase or Betterlight to use on a view camera would be a lot less of a unicorn to seek out I think.
