Having said that, one could still "potentially" automate EVC configurations using the vSphere MOB interface using the private vSphere API, but it may not be ideal and will require some "creativity" and custom coding to integrate with your existing automation solution. This particular limitation of the vSphere API is one that I have personally faced and have filed a bug with VMware awhile back. I am hoping this will eventually be added to the public vSphere API, so that users can fully automate all aspects and configurations of a vSphere Cluster.
Disclaimer: This is not officially supported by VMware, use at your own risk and discretion.
Step 1 - Connect to your vCenter MOB and traverse to the vSphere Cluster of interest (note the MOID will be different in your specific cluster).
Step 2 - Now replace the URL with the following while substituting the cluster MOID that you see in your browser:
and hit enter and you'll be brought to TransitionalEVCManager() method, you'll then want to click on the "Invoke Method". Once you do so, you should be returned with a task object and you'll have a link to something like evcdomain-cXXXX. Click on this and you'll be brought to ClusterTransitionalEVCManager.
Step 3 - From here you'll have have some basic evcState information which you can click on to see what the current EVC configuration is set to, guaranteedCPUFeatures and valid EVC Modes (the last part will be important for reconfiguring EVC)
Step 4 - Now let's say the cluster currently has EVC Mode set to intel-merom and you would like to change it to Nehalem, you would need to retrieve the key from the previous page, in our example it's intel-nehalem. Now, you need to click on the method link called ConfigureEVC_Task which is pretty straight forward, it just accepts the EVC Mode Key, enter the string and click on "Invoke Method" and now your cluster will be reconfigured if you go back to the evcState or look at your vCenter task. You can also disable EVC by using DisableEVC_Task
Note: If EVC is already configured in your vSphere Cluster, you can use the vSphere API to view it's current configuration by looking at the ClusterComputeResource's summary property. You just will not be able to make any changes or disabling EVC using the vSphere API.