It's been too long since I've used RIS but can't you just add the drivers to the install image? cf. http://support.microsoft.com/kb/315279
Putting a driver into a RIS image
Through dumb luck I happened to stumble upon a third KB article  in
which MS describes how to fix a bug in SYSPREP. The bug causes
multiple identical entries to be added to DevicePath. What is
DevicePath? It’s a registry value that contains a list of paths in
which Windows searches for drivers when it detects new PNP hardware.
Windows automatically performs an unattended installation of any
suitable driver it finds. The default value for DevicePath is
“%SystemRoot%\inf” and to be precise, it’s a list of locations in
which Windows looks for INF files. The INF files then tell Windows
where to look for the real driver files (SYS and CAT and the like).
Why don’t we just copy the raw driver files into a directory that’s
part of the image and then update the DevicePath accordingly? As the
driver directory belongs to the image, it will be copied to the
machine’s harddisk. When the machine reboots after the image was
installed, Windows should (re-)detect PNP hardware it doesn’t have a
driver for and look up possible driver locations from DevicePath. This
time Windows will find our driver and install it. Bingo! It’s kind of
like pre-staging the driver by copying the driver files onto the
target machine and telling it where to find the driver in case the
hardware for it is detected. How do you do this?
1.Create the directory \SERVER\REMINST\Setup\Language\Images\RIPrep\i386\Mirror1\UserData\Drivers\Broadcom.
Remember that RIPrep stands for the name of your image and Broadcom
signifies your hardware. These names are just examples and you will
need to adjust them to your requirements.
2.Copy the driver files into the newly created directory.
3.On the server or your personal workstation, fire up regedt32 if it’s Windows 2000 or regedit if it’s an XP machine. (My personal
workstation has Windows XP on it and so I use regedit.)
4.Load the Software registry hive that’s part of the image by selecting HKEY_LOCAL_MACHINE and choosing Load Hive on the File menu.
Navigate to \SERVER\REMINST\Setup\Language\Images\RIPrep\i386
\Mirror1\UserData\WINNT\SYSTEM32\CONFIG and pick the Software
5.Type RIS Image as the temporary name of the hive.
6.In the registry tree, navigate to HKEY_LOCAL_MACHINE\RIS Image\Microsoft\Windows\CurrentVersion and in the right pane double
click the DevicePath value.
7.Append ;%SystemDrive%\Drivers\Broadcom to the value.
8.Unload the hive by navigating to HKEY_LOCAL_MACHINE\RIS Image and selecting Unload Hive on the File menu. Don’t forget this step because
otherwise the hive will not be saved until you reboot your personal
9.Install the RIS image on a test machine.