Dan merged the external task support to the development branch this afternoon: http://git.beaker-project.org/cgit/beaker/commit/?h=develop&id=07869a2c0...
The design proposal has also been updated based on the merged implementation: http://beaker-project.org/dev/proposals/external-tasks.html
Note that the default beah harness is *not* compatible with the use of fetch URLs - for the time being, this feature is designed to be used solely with alternate harnesses (at least until we get restraint to a state where we're happy with it replacing beah as the default harness).
Cheers, Nick.