It is possible to do this with SiteScope using a Script monitor that points to a .bat script which calls a powershell script that invokes Get-ClusterSharedVolume on a remote host.
You will need a few scripts, and you need Remote Powershell enabled.
The Powershell script can output the free disk space, using the information available inside the objects returned by Get-ClusterSharedVolume.
The wrapper script that SiteScope calls can utilize Remote Powershell, so you don't have to copy and maintain the script on all of your Hyper-V servers. It will connect to a host and execute the local script against the remote host.