There is no safety concern with trimming .45 Colt brass to .45 ACP length so it will headspace on the case mouth. Actually a reasonably good idea. Extraction might be a bit dicey with the small rim though. This is the reason the Model 1909 cartridge had a larger rim, to be used in the Model 1909 revolver. Because of the larger rim it cannot be used in the Single Action Army.

There really isn't any "unsupported area" ahead of the rim. The case head is about 3/16" thick, so there is solid brass well into the chamber. At least with cases made since WWII!