Deploying XP Pro SP3 using WDS ignores sysprep.inf specified drivers

I am having trouble deploying Windows XP Pro SP3 and specifying OEM drivers.
In summary, I performed the following tasks:
Imported slipstreamed OS with SP3 into Deployment Workbench (V 5.0.1641.0).
Created a task sequence to produce a reference computer based on the XP OS.
Imported the required drivers into Out of Box Drivers.
Booted a client and ran the task sequence. This produced a WIM file in
Deployment Shares\Captures.
Added this image into WDS (running on W2K8EE).
Created a sysprep.inf file and stored it in
ImageGroup1\WINXPPRO\$OEM$\SYSPREP
Copied my drivers into:
ImageGroup1\WINXPPRO\$OEM$\$1\Drivers\NIC
ImageGroup1\WINXPPRO\$OEM$\$1\Drivers\Audio
ImageGroup1\WINXPPRO\$OEM$\$1\Drivers\Video

My clients with identical hardware to the reference machine install the
image great.
However, clients which require different drivers to the original image don't
pick them up from $OEM$.
I know this because the drivers aren't stored anywhere on the C: drive, the
DevicePath registry key is not updated, and the device manager shows the
devices with ? icons.

Sysprep.inf looks like below:
[Unattended]
OEMPreinstall=Yes
OEMPnPDriversPath=Drivers\NIC;Drivers\Video;Drivers\Audio
DriverSigningPolicy=Ignore


Any advice appreciated.