This is one of the OCS Escalation most recent works: It is officially documented as a KB: http://support.microsoft.com/kb/2029024. The same article applies to Communicator 2007 (not Communicator 2007 R2)
Are you sure that the Live Meeting client will mark packets with DSCP anyway? I think only the Communicator client and the MCUs, Mediation servers will mark packets.
On Windows XP it will, on W7 and vista its not supported/wont work