Hi,
The DSCP value is found in the IP header, which means the mapping can only be done, where you do an IP lookup. As such you would have to configure it on the router connecting to the MGX - either to an ATM interface or to the backplane (RPM).
What you possibly want to look at is "Support for Multi-VC on the RPM-XF"
http://www.cisco.com/en/US/products/hw/switches/ps1938/products_installation_guide_chapter09186a00800d8513.html#wp1106899
Make sure you use the Guide appropriate for your hardware/software combination.
Hope this helps! Please rate all posts.
Regards, Martin