If it is of any help, OCS also uses kerberos authentication. You can choose between one, the other or both.
Also, I believe if you use an OCS mediation server, you can bypass authentication entirely and use it as a simple sip gateway to route traffic between OCS and any other sip source (although I might be mistaken on this.)
If this isn't possible at present, do you see it as something that will be an option in the future? I would imagine if Exchange 2007 integration is possible, then OCS integration should not be too hard. There's no authentication settings required when integrating Exchange and OCS.