This might very well go against the EULA or ToS so do forgive me if this is a bad idea and could get someone banned for trying it.

Could you create a macro/UI mod that had the client act based on chat messages. For example, imagine the following.

You have 3 accounts with 1 character each account. First is a Mage, Second is a Priest, and Third is a Warrior who is being played by the primary player.

Instead of using something like KeyClone, what if the user would hit macros in his UI that would automatically have him issue out /party chat commands. These commands would then be interpreted by the Alt machines, to perform specific actions like "Mage Assist me", "Mage AttackChain1" or "Priest Heal Self", etc. And those actions would have some very specific meaning to each client/character. Note, as a safety measure, the Mage and Priest would only respond to the Warrior's commands. (i.e. me) Thus they can't get controlled by some other random user out to screw with me.

It has a lot of flaws, but it would be a neat design. Over all,your 100x better off using say KeyClone, but the idea tickles me as something that would be rather neat.