Dave,
I own an Old Army (not that I shoot it much, time constraints), and the loading lever is enough different that it would not be a speed reload. On the Remington, you drop the loading lever and pull the pin, change cylinders, replace the cylinder pin and lock the loading lever up. On the Old Army, you need to unlock the loading lever keeper, drop the loading lever and remove it and the ball rammer to get the cylinder out. Not impossible, but not as neet as the Remington.
Another issue is a source for a second cylinder for the Old Army. Ruger will not sell one to you without the revolver and the revovlers orignal cylinder being sent back for fitting, and they will not return the original cylinder. (source, the Ruger manual). So the only way to accomplish this is to find a second Old Army for the second cylinder. Then it is quicker to have two loaded revolvers.
The final caution ( and probably Ruger's main reason for the above) is that you would probably not want to handle a loaded and capped cylinder by itself. While it may be a slim chance, a cap could receive enough impact to go off, resulting in a ball ending up someplace you did not want, and a cylinder bouncing around some more as the result of reaction. I belive that people that swap out cylinders on the Remingtions do not cap until after the cylinder is in the revolver.
Neal