Many people have been working on this exciting subject. Jun Tsai has a Tapestry 5.0.18 application running http://ganshane.appspot.com. It looks like he took the Tapestry 22.214.171.124 quickstart archetype and just shifted the version number down to 5.0.18.
Tapestry 5.1 deployments haven't been working, because of the shift in XML parser from generic SAX to Woodstox STAX. An issue has been added, and the Application Engine folks are looking to add the necessary classes to their "white list", but it's not clear when such support will be added.