The Active Directory Module for Windows PowerShell, which is included with Windows Server 2008 R2, can be used to administer Active Directory Domain Services (AD DS) objects, including computer ...

Creates the local and domain PSCredential objects to authenticate to the remote computer; Removes the computer from the domain by using the Remove-Computer cmdlet—providing the local credential as ...

Actually you only need to install the root CA certificate on the non-domain joined machined so that they trust the certificate on the RDP server. You can do that pretty easily via PowerShell.