First you must be using HDL patched with the ToxicOS 0.2 core.
Then using my Star Wars Battlefront technique
Modify the topline of the SYSTEM.CNF to read:
BOOT2 = cdrom0:\BO3NETGM.ELF;1
Enable MODE 3
NOTE:- This will only allow to play multiplayer LAN once modified, single player will NOT load after this!!
Hope this helps....