Update now was a specialized message to the agent in the 4.0(?) time frame to give an administrator some amount of remediation capability. It's been pretty much entirely superceded in the 4.6 timeframe with the ability to run any client task on demand (not just update tasks). But you should be able to do exactly what you want using the 'clienttask.run' command.
Silly me! And i can even run client tasks not assigned to that specific system! Thanks for leading me on to the right track...