I don't know the rules for epic spells, but maybe you could simply break apart some of the difference between maneuvers and spells and try to work out some sort of epic maneuver system. At epic levels, it's not unfeasible that warriors have occasional/regular/frequent "strokes of genius" (pun intended) and initiate some sort of awesome maneuver in the middle of a fight, which they may never be able to replicate ever again.

Of course, initiating such a maneuver would happen in the middle of combat, so very quickly, and are almost purely your own prowess, so no time and money expenditure, and XP expenditure seems unlikely. They'd have to be weaker than epic spells still, but they also wouldn't have to be repeatable unlimited times/day.