Much to my elation it turns out the 2012 RDS License server and its 2012 CALS are compatible for use with Windows 2008 server.

I can confirm this works and have tried it in my lab.

2012 RDS Licensing server with 10 x 2012 RDS CALS with a 2008 RDS pointing to the 2012 license server. Connections worked and licences were checked out.

Thank you Microsoft for having the foresight to make this easy!

References: 

http://social.technet.microsoft.com/wiki/contents/articles/14988.rds-and-ts-cal-interoperability-matrix.aspx