I don't know of any way to get around the "unsupported operating system" nor do I know if support for 2012R2 Essentials is coming soon - if license costs are the issue and why you can't use another Server product how about Windows 7 Professional/Enterprise sp1
See QuickSpecs for details: http://www8.hp.com/h20195/v2/GetDocument.aspx?docname=c04154481