I agree with Draconis.
Something important to understand is that adding a new direction implies that it is distinct from every other direction.
It’s not clear whether you intend for regular directions like south to be meaningful at all in this example. Do you?
If not, then you can just replace the portion of the Standard Rules defining directions to attack the issue at its core. Here’s a 6M62-compatible method:
Section Nautical Directions (in place of "Section SR1/4 - Directions" in Standard Rules by Graham Nelson
The specification of direction is "Represents a direction of movement, such
as northeast or down. They always occur in opposite, matched pairs: northeast
and southwest, for instance; down and up."
A direction can be privately-named or publicly-named. A direction is usually
publicly-named.
A direction can be marked for listing or unmarked for listing. A direction is
usually unmarked for listing.
A direction has a direction called an opposite.
Include (-
has scenery, ! class CompassDirection,
-) when defining a direction.
[might want articles here]
There is a direction called fore.
There is a direction called aft.
There is a direction called port.
There is a direction called starboard.
[per original]
The up is a direction.
The down is a direction.
The inside is a direction.
The outside is a direction.
Fore has opposite aft. Understand "f" as fore.
Aft has opposite fore. Understand "a" as aft.
Port has opposite starboard. Understand "p" as port.
Starboard has opposite port. Understand "s" as starboard.
[per original]
Up has opposite down. Understand "u" as up.
Down has opposite up. Understand "d" as down.
Inside has opposite outside. Understand "in" as inside.
Outside has opposite inside. Understand "out" as outside.
The inside object translates into I6 as "in_obj".
The outside object translates into I6 as "out_obj".