Rather than SCOM pushing agents, we want to bake the 2016 agent into the server image, and then have the MGMT group assigned during deployment (ideally, via PowerShell). Is there a cmdlet for this?
This can be done via command line when doing the agent install
Have a look here https://technet.microsoft.com/en-us/library/ee424297.aspx
You can chose either to do AD integration config so agents will find the management servers themselves, or manual agent deployment (https://docs.microsoft.com/en-us/system-center/scom/manage-ad-integration-agent-assignment).
I used Martin Erhnst’s script for this before and it worked fine: http://adatum.no/operationsmanager/script-to-add-scom-agent-management-group .