There's a hell biome page in mystcraft. Maybe have a sort of technology barrier where you have to trade a glittering queen for the descriptive/linking book? That way the player must already have a stable source of free gold before getting the Nether.
edit: I see what you mean. Using MCedit, you can just select all and fill with air, then surround it in a bedrock box. Another option is to simply make a hell biome section in the Overworld. maybe one full chunk, where the floor is replaced with dirt anticovers/ covered with covers?