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!