After googling this a bit, I'm starting to come to the conclusion that this simply isn't possible with Powershell. All the code samples I've found use Active Directory, something as an OEM that I will never use.
I suspect that there may be some confusion between the terms "Active Directory" and "ADSI" (Active Directory Service Interface). You can use ADSI as a programming API even in a workgroup or standalone computer environment, via its WinNT Provider. That's how the module that Øyvind linked to works.