I think having all these issues with the BB, noise, locking up etc, the dealers have been second guessing themselves as to what it the correct fill level. I believe they're just afraid to take a stand. This boils down to MMC not being definitive in the maintenance manual and clearly stating the proper level.
But when it comes down to it, noises and issues aside, the fill level is always the side fill plug hole.
Lets say you have a standard 4-wheel vehicle with a solid rear axle and third member, there is a fill plug about the level of the mid pinion shaft range. They also have a vent on top. Who would "ever" consider filling the entire third member with oil through the vent??? They have the very same ring and pinion design, so why would the BB be any different.
When there is no side fill plug, I've never seen one without a dip stick attached to the bottom of the cap.
In most cases, seals are really not designed to be flooded with oil, but to retain oil as it gets splashed in their general area.
Again with all the issues, I think that anything was open to being tried, including filling the entire BB with oil.
Keep in mind the amount of extra parasitic drag when this thick of oil is constantly being pushed around by the ring/pinion when the BB is completely filled to the brim.
As has been pointed out a few times, I have no skin in the game, but I would only fill to the level of the side fill hole and then worry about other means to quiet it down. I wouldn't even think about filling higher than the side. I might fill through the vent hole if it's easier to access, but when oil runs out the side hole, that's full and I'd put the plugs back in and call it done.
This is a new Honda Goldwing GL1800 final drive assembly. The large hole on the side is the fill hole, the lower is the drain and the upper chrome top hat is the vent.
You can see that when FULL, the level is just about 1/3 up from the bottom of the case. The Honda final drive is setup very similarly to the BB, in that the pinion is centered in the ring gear, unlike most conventional car third members where the pinion shaft is about a third the way to being centered.
