I've already asked a question about this in Unix & Linux SE and I so actually use xdotool.
The idea is that the mouse goes at the left of the left screen when it reaches the right of the right screen and vice versa.
But I recently saw the mouse.coords table in the awesome API, so I'd like to give up xdotool to set mouse coordinates as do xdotool with this possibility.
I suppose I should add a signal to the root to know when the mouse is on the edge, but I don't konw how to do this...