I believe this is meant to hint upon the suspected Et'Ada the dwemeri studied within the present patterns of the Magna Ge's trailings.
If this means that some Et'Ada essence is trying to escape the Mundus or Oblivion for the Aetherius or http://images.uesp.net/7/73/Rg_dwemerstarmap.jpg, I'm not sure.
However mad this sounds, I believe there is some sense in it. I think the dwemer were trying to identify the essence that the spirit embodied.
The letter is left as a footnote to a specific Et'Ada. This Et'Ada name which the dwemeri refer to could be from any religion on Tamriel.
In attempt to conceptualise to their correct findings, I'd believe the dwemer would look across the continent for the right identity to each Et'Ada.
The Et'Ada's name is then translated to dwemeris. Shortened down to a single letter after that, for simplification and convenience's sake.
In this mind I constructed a alphabetical, yet symbolic supposed order to the astronomy translated into Et'Ada concepts, using expanded localised sourcing.
Astonomy to Et'Ada:
The Thief - A - Akatosh / Alduin
The Ritual - B - Boethiah
The Lover - D - Dibella
The Lord - E - Ebonarm
The Mage - F - Frandar Hunding / HoonDing
The Shadow - G - The Greedy Man
The Steed - H -Hermaeus-Mora
The Apprentice - I - Ius
The Warrior - K - Kyne
The Lady - N - Namira / Nerevarine / Nocturnal
The Atronach - R - Ruptga
Unknowable letter meaning:
The Tower - ? - Sai / Nocturnal / Y'ffre / Trinimac?
Not shown:
The Serpent - ? - Sheogorath
Possible reason: The depiction of Sheogorath, as well as The Serpent's embodiment being described both as unknowable.
They do not appear on the orrery I think for Sheogorath is described as a 'Sithis (void) shaped hole', you'd never actually see the Et'Ada.
If none of this makes any sense to you, I'll try to simplify it the idea: the dwemer found a number of ethereal bodies, Aedra, Daedra, near the stars.
They couldn't describe their qualities in their language, dwemeris, but discovered other races could. Building numerous fortresses outside
their usual locale, they discovered these spiritual concepts from other languages, races or beliefs. And used them to collectively confine their findings.
Now this may sound mad, like I've said before. It could even be too extensive for the dwemeri, which I have considered. It may even be that some
crack pot has made a better idea that my own speculation on their use of dwemeri alphabet in the orrery. However, I'd still like some positive feedback.
I imagine there is some crazy flaw in my idea and some Captain Obvious is going to come to point out. So I shall be waiting!