Err, well, messages can be received by a Client, but apparently not a Designer:
Messages cannot be received within a Designer.
Ok, hmmm. Any way to automate this process then?
I'm out of ideas for now 
I'm not sure why you would want to automate this. I can't imagine any user with an open Designer is going to be super happy that you forced an update on them rather than letting them deal with it when they're ready to save their work, assuming the resources changes enough to warrant needing intervention.
I'm trying to display a window to a user saying 'changes detected to X project, would you like to update with them immediately or wait' and then autoupdate if they accept.