Hello,
The Windows 10 KMS client keys are now available on the TechNet page with all other modern operating systems :
Windows 10 Client KMS Keys
Modern Windows KMS Client keys
Enabling Windows 10 activation on down level operating systems
Microsoft published a patch for Windows Server 2012/2012R2 to act as a KMS host for Windows 10 :
https://support.microsoft.com/en-us/kb/3058168
A patch will come for Windows Server 2008R2, no ETA though :
http://blogs.technet.com/b/askpfeplat/archive/2015/07/20/mailbag-hi-mom-issue-12.aspx
Update : The down level patch is available :
https://support.microsoft.com/en-us/kb/3079821