It is possible indeed. Simply set up local users on all workgroup machines with the same username/password, or, if the Administrator accounts all have the same password, use this account. Make sure you grant the account you will use WMI read access on each machine.
On your splunk instance performing the remote WMI connections, create a local user with the same username/password as the workgroup machines, and run splunk as this user. Splunk now will be able to remotely query the workgroup machines via WMI.
