Start a project on sourceforge and write this yourself. You can start with tftping a shell script on a schedule. Then have the shell script ran at the end of the schedule which then tftp's rpms and installs them.
To my knowledge you won't find anything like GPO's for linux for cheap and free.
If you want to spend money...
http://www.vintela.com/ They seem to have been bought by quest software. They have a product called Vintela Group Policy. Not sure what it has been renamed to.