That would be clean, but perhaps get one from class? As feats are still so scarce for melee. That would fulfill what Cosi says, about the default being an adept of one discipline, but then with the option to branch out.
Edit: They could be formatted as tactical feats, potentially. Noncombat equivalents to the combat tactical feats the disciplines already have.
Or they could just be utility maneuvers. I don't actually think any but a couple of the things we've mentioned would be overpowered as at-wills.
I know Eldan mentioned this earlier up, but it seems like most of them could be stances, too. A Shadow Hand stance that lets you travel as a wraith through the plane of shadow, crossing great distances in the mortal world, Diamond Mind meditation stance that gives you benefits if you spend one hour in it, the earth-shaping Stone Dragon stance Eldan mentioned and so on.
Setting Sun is about using cleverness to overcome larger opponents. So I think their shtick should be cleverness and ingenuity. That's not a trait covered by the other disciplines, either.
I think this takes it too far from the purely mechanical, leverage-based heart of the discipline.
But I'm reminded of how in this novel, Theseus, who again is *so* Setting Sun, uses a lever and fulcrum to lift a giant rock. Perhaps SSers have almost MacGyvery resourcefulness? It would fit the theme of the "scrappy hero" that seems tied to the discipline, for example in its legacy weapon story. They could get skill bonuses, create effects equivalent to Grease and Glitterdust, have some effect equivalent to shapesand (refluffed as macgyvering impromptu materials). If Shadow Hand is the "stealthy assassin" side of rogue, then Setting Sun is the "skillmonkey" side. They can break out of jail, or you thrust them in the driver's seat and they learn how to fly the plane on the go, or they figure out how to work the ancient machine.