In fact, yes, it is somehow possible.
Windows 7 includes a "boot to VHD" feature. So, it is possible to install Windows Server 2008 R2 (64bits) inside a VHD that resides on the Windows 7 file system. This system will be available as dual-boot on system startup.
When booted on Windows Server 2008 R2, it is possible to enable the Hyper-V Server Role and everything works perfectly.
In order to set this up on your Windows 7 workstation:
a) Boot on the Windows Server 2008 R2 Installation Media
b) When the language selection dialog appears, hit Shift+F10
c) In the command prompt, run a series of command to create a virtual hard disk.
create vdisk file=c:\windows7.vhd maximum=25600 type=fixed
select vdisk file=c:\windows7.vhd
Then, you can proceed with the installation.