It is possible to set extraConfig values for a new VM in the VirtualMachineConfigSpec prior to CloneVM_Task.
The config spec is built by this method https://github.com/ManageIQ/manageiq-providers-vmware/blob/master/app/models/manageiq/providers/vmware/infra_manager/provision/configuration/container.rb#L2-L52
It would be beneficial to allow the user to provide extraConfig key=value pairs to set on the new vm