I have not tried it yet. I just finished my new MCE that has front 1394, so once I do, I'll post the results.
There's nothing magic about WHS that isn't already in Windows -- it just exposes some CIFS shares to which other machines can connect. Even if you map a drive, they still look like network drives. So, it's just a matter of whether the Roxio software on the Media Center exposes only local drives, or a list of local and network drives.