Just found a great set of utilities to assist in installing drivers into SOEs. It is called Driver Install Framework Tools and is available from Microsoft. One of these tools is a utility called DPInst.exe that will install any drivers it finds within the current working directory (or sub directories if specified in an optional xml file). This means that it is possible to create an XML file that contains all paths for C:\Setup\DRV and launch DPInst.exe and it will install all drivers that are compatible. It also has switches to install a driver even if it is not the best match.
http://www.microsoft.com/whdc/driver/install/DIFxtlsdwn.mspx?