Hitting the Wayland mailing list were a set of patches by Scott Moreau that introduce a ping event for Wayland as well as support for setting the brightness and saturation for the surface shader.
The basic premise is that the Wayland compositor can ping clients and if the client doesn't respond to that ping request with a pong, to the point that a time-out is reached, that application is likely hung. If a Wayland client is determined to be hung, the surface (interface) can faded via adjusting the saturation and brightness to indicate the hang to the end-user. If the client later begins to respond, the action can be undone.
This has long been a Wayland TODO action item. A simple set of three patches provided this support. There hasn't been any developer comments on the patch-set so far, but it's fairly straightforward.
Meanwhile, there's still a lot of other activities going on with Wayland as summarized earlier this month.