I don't blame you for not wanting to install the client on a W2K server. If you need the Banyan Client for W2K I think I still have it. I can mail it to you for free as long as it is legal to do so. You can't buy anymore so I should be able to give it to you for free and still be legal.
Solutions:
First, you could install the Client on a NT 4.0 Server and still be part of your Active Directory domain if you have one in Native mode.
Second, install W2K on a server, make a full backup and store the backup in your desk draw. Now, install the Banyan client, migrate your data over under Banyan with ARLs to a seperate partition on that server. Figure out the NT permissions you want and set them. Next, have users try access the data under NT. If they can you are half-way home. Now, reboot the server and wipe the the system partition and leave the data there. You can wipe it using ERD command boot disk or reinstalling NT. Just reinstall W2K and do a full restore from the tape in your draw. As for the permissions, don't worry. As long as you used global groups/accounts they will still be there. You see, NT/W2K use NTFS which actually holds the permissions/SIDs even if you reinstall the OS. Now you have all of your data and all of the permissions and no signs of Banyan.