Hi James,
Yes, this would be a good approach. There's nothing in the gwt plugin that currently supports server side push events down to the client.
This is something that is commonly done in many JS side apps, and I've found that atmosphere is an excellent framework to use.
The grails plugin ecosystem does have some help on hand for this.
I'm not sure if there's a way to address a particular browser from the server side, I don't believe so. If you don't need to do that, then those two plugins, plus possibly GWT/ atmosphere on the browser side should get you a long way to having browser<->server eventing in a pretty minimal amount of code.
Best,
David.