This is taken from Niantic’s initial announcement on Elite TMs.
The wording here states that Elite TMs will allow Mons to learn moves previously only available via events such as community days and raid events. Omastar’s legacy moves are neither of these things, and aren’t tied to an event of any kind, so I would assume the answer is no, an Elite TM won’t let you access those moves.
If anything, I think it’s more likely that Mons like Omastar will have their legacy moves returned in a future moveset update. They’ve already done this with a handful of others (Gyarados, Dragonite, Snorlax, Magneton, Haunter, Hypno, etc).