Like Applets (that read codebase from the html page), read the codebase from the related JNLP file (using Web Start) for deploy.
So in DesktopApplicationContext, make a private static method that tries to get the codebase from Web Start BasicService, and if successful, set the webStartEnabled flag (to be put in ApplicationContext) ... and if possible set the origin property with the retrieved codebase (maybe only when origin is null).
Verify (with other Pivot Developers) if it's right to require Web Start jar even at compile time (like was some time ago), because some casts are even required in this new code (so all dynamic doesnt' makes sense).
Then, revert changes to build.properties and build.xml just committed as
PIVOT-733 (and maybe even demo.jnlp.xsl to remove the root tag), and maybe make a little change even in LargeData.java (if needed).