Aaannnd just after posting this I did a bit more searching on "ext.policy.activedirectory.system.id" which led me to the Custom Property Dictionary.
Custom Properties E lists: "ext.policy.activedirectory.orgunit" - The organizational unit that you want to use rather than the organizational unit in the current Active Directory policy. Overrides the ext.policy.activedirectory.system.orgunit value that is specified in the Active Directory policy.
This looks like exactly what I'm after. I'll give it a shot!
Adding this property and populating it with a distinguished name did the trick. The computer object was created in the correct location.