I think I may have found the actual reason they don't recommend it. I have to retract my assertion that their motivation was financial, I stand corrected. I looked in a couple of reloading manuals at the case and overall cartridge dimensions to try to find some clue as to why firing .45LC in a .454 would create a problem. In nearly every case of a "magnum" cartridge, it's smaller parent cartridge case was just lengthened, usually about a tenth of an inch, in order to create the new magnum case. However, for some reason when Dick Casull designed the .454, he decided to make the case diameter .478" instead of the .480" of the .45 Colt parent cartridge. His reason for doing this probably had something to do with the cartridge's pressure curve, and resulting velocity. He must have considered the gain significant enough to warrant making such an odd change. At any rate, it is different, by .002" two one thousandths of an inch. Not much, but it does mean that the SAAMI specs. for the chamber diameter would be smaller for the .454 than for the .45LC. This would cause some case-sticking issues, without a doubt, and is most likely the basis of their recommendation against shooting .45LC in the big Casull.
Ok, that's the problem, now how do we solve it? Well, the extra cylinder solves it very nicely. The downside is you have to send your gun back to FA and it's expensive. There is another option, though. Identify the .45 LC ammo you plan to shoot in your Casull and resize it using the .454 Casull full-length resizing die. Use the other two dies from the .45LC set to finish the reloading. What this will do is give your .45LC cases a diameter of .478" just like a Casull round. They should be no more prone to sticking than the .454 cases are. The downside (there always is one) is that resizing your brass like this will probably shorten the life of the brass, I don't know how much, but .45LC cases seem to be prone to splitting anyway, at least the Remington cases are - in my experience. But ... $382 dollars will buy a lot of .45LC brass!