I don't see the point in having a "seamless" Linux guest. Everything is displayed using X anyway, so you can "ssh -X guestComputer", and it will forward any new windows created from that terminal to your screen. Doing that will give you a truly seamless experience (minus sound) with most applications. There are a few exceptions, firefox may detect if its already running on your screen and re-use your local process. Since thats a basic feature of X you can do that with real remote computers too. Sometimes I'll configure things on my wife's computer without interrupting her, and when I'm done I'll just tell her the program is ready and configured.