Has anyone encountered the use of Omega to represent equilibrium?
I would guess $Ω^m$ notation is a personal invention and I couldn't find any sources that would standardize it.

The choice of these symbols for abbreviating "equilibrium" may be justified that the glyph "Libra" ♎ originates from the Greek letter Omega Ω.
Further, from Merriam–Webster Online:


  Equilibrium contains a root from the Latin libra, meaning "weight" or "balance". As a constellation, zodiac symbol, and astrological sign, Libra is usually pictured as a set of balance scales, often held by the blindfolded goddess of justice, which symbolizes fairness, equality, and justice. Equilibrium has special meanings in biology, chemistry, physics, and economics, but in all of them it refers to the balance of competing influences.


Uppercase letter "m" is probably because $Ω^m$ is a shortened form (contruction) of a single word that ends in the same letter: equilibrium.

$$\underset{Ω}{\underbrace{\text{equi}\underset{♎}{\underbrace{\text{libr}}}\text{iu}}}\underset{^m}{\underbrace{\text{m}}}$$