There are various methods to update the PnP directory with the path to the INF file. Insure that you have no video driver captured in your image file (uninstall any dirvers and leave the default VGA drvier).
In the UNATTEND.TXT file add to the Unattend section a OemPnPDriversPath setting. Include your PnP paths. I used a set of predefined path roots. They are based off of your install drive. Prune / modify to your needs. The fun thing is that you could customize to each hardware model. This should work for everything except the IDE/SATA/SCSI controller which needs to be in txtsetup.oem and the appropriate directory.
The sysprep,ini and winbom.ini have similar functionality for setting pnp paths. I just prefered setting all the PNP roots in the original UNATTEND.TXT file.
Note: Some drivers just do not like this method. Running the full setup silently later is the only method in those cases.