The hole that Keith is talking about is nowhere near the fill plug, and is found even on cars that did not originally have a fill plug (like mine). I'm not positive what it was for, but I don't believe it was for the fill plug.
I don't like having wires through it either, though, so mine has a blank plug in that hole and the wires are routed through the engine compartment.
PS, perhaps I should explain that earlier cars (before TS50,000) did have a hole for filling the gearbox, but it is over on the right side and near the top directly above the dipstick. Prying out the plug and filling through the dipstick hole was the only method available, as those cars did not have the pipe plug on the side as later cars did.