I did a post a year or so ago about setting up TFS Labs and mentioned command
C:\Program Files\Microsoft Team Foundation Server 2010\Tools>tfsconfig lab /hostgroup /collectionName:myTpc /labenvironmentplacementpolicy:aggressive /edit /Name:"My hosts group"
This can be used tell TFS Lab Management to place VMs using any memory that is assigned stopped environments. This allowed a degree of over commitment of resources.
As I discovered today this command only works for SCVMM 2010 based system. if you try it you just get a message saying not support on SCVMM 2012. There appears to be no equivalent for 2012.
However you can use features such as dynamic memory with in SCVMM 2012 so all is not lost