Yes, but quite dangerous to carry around a capped loaded cylinder. It works in the movies and I think the principle character in that Hell on Wheels series did it.
Thing is the caps, modern or original period are way to sensitive to roll around unprotected on the nipple of a loaded chamber outside the gun where there is no blast shield to protect it. One good whack and it will discharge. I have seen one go off in the capping process in the gun when the shooter was trying too hard to seat a primer.
Sorta like putting your hand actually on the handguard of the cap and ball revolver rifles, keeping a loaded chamber under the hammer of a real SAA, smoking a cigar while loading your cap and ball, loading undersized balls in a C&B cylinder, I think it was likely some tried and discovered it was a really bad idea.
There is no doubt that having a spare cylinder loaded and ready to cap for an 1858 Army would lead to a faster reload, and without most of the risk if you kept it uncapped and covered and in a pouch. To just cap it and slide it in would be quicker than a reload while in the gun. But I would just as soon ride around with a cocked loaded revolver in my holster as a capped loaded spare cylinder on my waist like that.