So if I understood you correctly, you would want to push all the contents of a TabSheet to the client in one request, so that switching between them wouldn’t cause a server round-trip.
Could you describe your use-case a bit, why you’d need this? IMO this would only be beneficial if the contents of the tabs are fairly simple, so it doesn’t add too much traffic for the first render.
Anyhow, modifying the Tabsheet to work this way shouldn’t be too hard, it might not even require any GWT coding. Modifying the server-side TabSheet component’s paintContent method might be enough.