First I know it would be in breach... but I like looking at things like this. :S

The only way you could make it work would be to hook into the memory or change models; PvE mobs have too large a range of shape, size and color scheme to make it feasible to look for something on the screen and have an automated process follow/attack it.

If you look at the semi professional botting solutions they almost never use a melee character, for precisely this reason, and when they do its generally for a specific follower that just loots or something. So to make it work, if its possible, you'd need to hack the game files to change models; create the software that could interpret the changed models and direct the character; which would need to hook into the memory to work effectively; and run some sort of masking software to hide all the shenanigans from Warden...

And when you got caught you'd lose the botted account and most likely any other accounts associated with it, eg the main who always plays with the follower. And make no mistake you would be caught, even just running it in instances doesn't mean you're 100% safe whatever they say on the botting sites.

Now one of the comments often levelled at dual/multi-boxers is that we have no lives... not so in fact its people who use bots/PLing services to play the game that have no lives, I mean why pay to play a game that you don't in effect play. You might as well just send the money to my paypal account knowing that I will play the game for you...