XMonad + Synergy - change window border state when the pointer leaves the computer

Question

My current XMonad setup happily highlights the currently focused window with a nice green border.

import XMonad.Layout.NoBorders
...
defaults = defaultConfig {
    focusFollowsMouse  = True,
    borderWidth        = 4,
    normalBorderColor  = "grey",
    focusedBorderColor = "green", ...
...
myLayout = avoidStruts $ smartBorders ( ...

However, when the mouse/focus moves off one of the multiple screens onto a neighbouring computer due to the use of Synergy, XMonad doesn't react - I'd like it to mark the window as unfocused, because all input now goes to a different machine.

Can anyone please suggest a mechanism I can use for this?

I don't know how to get the name of the 'currently selected computer' from Synergy, but let's assume it is possible (I can always modify the source for Synergy) - an approach for changing the focus in XMonad is what I would like most.