I found how to customize Live Meeting invitations in the hosted version of Live Meeting Professional but it shows on the feature comparison list that is is not available in the OCS 2007 version. Seems like there should be an XML file somewhere for this but it appears to be hardcoded in the product and language packs.
Live Meeting Feature Comparison:
http://office.microsoft.com/en-us/livemeeting/FX102407251033.aspx
(note the section engaged attendees that shows custom invitations)
You can see the instructions for customize meeting invitations:
http://technet.microsoft.com/en-us/library/cc836521.aspx
(again, only applies to hosted Live Meeting Pro)