While you perform an attack, you may spend 1 focus , hit , or crit result to look at the defender's facedown damage cards, choose 1, and expose it.

Is this ability locked into the “modify attack dice” step in the rules reference?

Yes.

Spending results is a modification, and dice modification can only happen in the modification steps (unless noted otherwise)

(rrg p9)

The "Attack" section on p. 4-5 isn't as helpful as "Dice Modification" on p. 9. This both specifies that Spending a Result (such as Thane's ability) is dice modification, as well as locates all red dice modification in the "modify attack dice" step.