This might be kind of difficult, perhaps not impossible. If you have your cluster set to automatic, this will be pretty hard to do during real time unless you setup some type of script to watch for vMotion activity through the tasks in vCenter. If you set your cluster's DRS to semi-automatic, where DRS will provide a recommendation but you need to manually execute the recommendation to actually initiate the vMotion, then it might be possible.
You may want to take a look at the recommendation list http://www.vmware.com/support/developer/vc-sdk/visdk400pubs/ReferenceGuide/vim.ClusterComputeResourc... to see which VMs should be migrated and from there you'll have access to the VM to pull whatever stats you need.
=========================================================================
William Lam
VMware vExpert 2009
VMware ESX/ESXi scripts and resources at:
VMware Code Central - Scripts/Sample code for Developers and Administrators
If you find this information useful, please award points for "correct" or "helpful".