Can I enforce mail rules server-side in Exchange 2007?

Exchange 2007 offers Message Transport rules to filter messages based on a number of conditions. One of these conditions is the HeaderContains condition which should do what you want.

To get it to send the mail to your Junk folder, you could set-up SCL to send messages over a certain threshold to the junk folder, then get the message transport rule to stamp an SCL value on the message, higher than that threshold, should that header exist.

Details of what filtering Message Transport Rules support is available here.

Details of how to enable Message Transport Rules can be found here.