There is a workaround for discovering Windows devices as Network Devices in SCOM 2012. There's a link that explains this:
http://social.technet.microsoft.com/wiki/contents/articles/23549.how-to-discover-windows-computers-using-snmp-via-scom.aspx
Unfortunately, this doesn't help since the SCOM 2007 OpsCenter MP requires the subject Windows Server to be discovered as a "Legacy Network Device". You can't discover new network devices in SCOM 2012 as "Legacy Devices", only under SCOM's new network device model.
In short, the SCOM 2007 OpsCenter MP doesn't work, unless you rewrite the entire MP to use the new SCOM 2012 network device model.
Symantec really needs to update this MP, IMO. Even a simple update that doesn't change the function of the original MP, but changes the network device model would be helpful.