I would mark them as guests using a custom property then create an alerts specific to that property and the alert threshold.
Alert when all the following are true
* CPU greater than X
* Is Guest equal to Yes
Wouldnt that still require the ability to poll the guest itself? From within the VMWare Vsphere client you have the graphs for Host CPU active vs Host CPU Available I want an alert whenever Host CPU is equal or greater to 60% of Host CPU Available