Fanning the Hammer on a single-action revolver
 

Can this be done with any single-action revolver, or only on older ones that don't have modern internal safeties?

I think you can fan most revolvers with an external hammer, not just SA ones (although it is pointless on DA/SA revolvers I think it is still mechanically possible). You really shouldn't do it though as it is very hard on the gun. Because you are driving the hammer back at much greater speeds than normal cocking it basically increases the forces on all the internal components greatly accelerating wear.

I remembering hearing that cowboy trick shooters use modified strengthened revolvers for fanning. Even with this they still need three revolvers: one to shoot, one as a spare, and the third is being rebuilt at a gunsmiths.

I'm told that they also use special lightweight aluminum or titanium blank firing cylinders.

Yeah, trick shooters and quick draw artists who do fanning tend not to use standard guns. The fanning causes great wear on the components and risks causing the gun to go out of time. I've even seen a double-action blank gun that was broken completely by someone fanning it.

Would you be able to fan a revolver with a transfer bar, like a Ruger Vaquero?

It should do, the transfer bar is simply a piece of metal that is linked directly to the trigger. When you pull the trigger the transfer bar slide upwards behind the firing pin (if it wasn't there the hammer wouldn't reach the firing pin hence wouldn't fire), assuming you continue to hold the trigger all the way back the transfer bar would stay in the firing position. There is no disconnect or anything like that that means it goes back to the safe position after firing and the trigger needs to be reset to get it back to the firing position.

You can't fan a double action revolver since the hand rotating the cylinder involves movement of the trigger as well as the hammer. Single actions rely on the hammer to do everything, and so you can just hold down the trigger and fan away on the hammer as it rotates the cylinder and falls, not interfacing with the trigger or sear components.

Double actions are more complicated. While you can cock the hammer to put them into single action mode and pull the trigger, the trigger needs to reset fully before the hammer can work with the hand to turn the cylinder again. In other words, when you hold down on the trigger and fan the hammer, the hammer can't move the hand and turn the cylinder, so there's no more cartridges getting in front of the firing pin.

I guess in theory, one could fan the gun if they reset the trigger into double action mode each time, but it's so tedious, especially when you have a trigger that cocks the hammer for you to achieve rapid fire anyway. Add to the fact that the fanning would cause considerable awkward wear on the sears and internals, perhaps even the cylinder, and that rapid fire is not generally condoned, and fanning DAs becomes all but pointless.