In theory you should be able to write a stub ant file that uploads something using that ant task to a build (even if it's complete). Can you try that to isolate things. Also, you can always start the ant process in debug mode and connect remotely via the Eclipse remote debugger to see what it's actually stuck on.

(eg, use the -Xdebug -Xnoagent -Xrunjdwp:transport=dt_socket,address=8787,server=y,suspend=n vmargs to connect)

Have you checked that you aren't publishing a 4GB file ;)

If this doesn't work, please log a work item against the build team.

Cheers,
Jean-Michel