Hello,
While I am certainly not authorized to speak for the manufactures, my understanding is there are three reasons the manufactures do not recommend shooting the shorter "sister" cartridges in the longer chambers. (this also applies for other calibers such as 38 special & 357 mag in the 357 maximum, 44 special & 44 mag in the 445 Supermag, 41 magnum in the 414 supermag, ect)
1) As mentioned in the other posts, chamber residue buildup from shooting the shorter cartridges can cause high pressures if the residue is not removed before shooting the longer cartrdges. (Easily fixed- clean your gun!)
2) Increased lead spitting from the barrel cylinder gap due to a greater possibility of the bullet not being lined up properly with the forcing cone due to the longer "jump" the bullet has to take through the cylinder.
3) The danger of handloaders trying to load 45 LC brass to 454 levels (or 44 special or 44 mag to 445 supermag levels, ect. ) because the gun can "handle it" - (Yes but the brass can't)- This is the main reason they don't recommend it. If you keep your 45 LC loads at 45 LC levels, you shouldn't have a problem.
As you can see, all these reasons are to protect the manufacture from liability, they have nothing to do with the capabilities of the gun. If you use 45 lC loads, clean your cylinders before shooting 454 cartridges, & observe proper shooting safety (glasses, gloves, ect.) You should be fine.