Can Heimerdinger be triggered by Nasus, Curator of Sands legend ability when you play a unit with 7 or more energy cost? Heimerdinger's card has a tap symbol physically, but the text on it does substitute it for "exhaust," which Nasus does do.

No. Nasus's triggered ability cannot be copied or triggered by Heimerdinger.

Two reasons:

1. Nasus's ability is a triggered ability, not an exhaust ability. Curator of the Sands reads: "When you play a unit, gear, or activated ability with Energy cost [7] or more, you may exhaust me to ready up to 2 runes." This is a triggered ability (starts with "When you..."), not an activated ability with the format [Cost]: [Effect]. Heimerdinger only copies exhaust abilities — defined as activated abilities with the Exhaust symbol [E] as their cost (FAQ #1575). Nasus's ability uses the word "exhaust" but does not use the [E] symbol, and is not an activated ability.

2. Even if it were, Nasus says "exhaust me." The cost is exhausting Nasus himself, not Heimerdinger. Heimerdinger cannot substitute himself for that cost.

Heimerdinger is unrelated to Nasus's trigger. When you play a 7-cost unit, Nasus's ability triggers independently, and only Nasus himself can be exhausted to pay its cost.

(FAQ #1575; Rule 383 — Triggered Abilities vs. Rule 376 — Activated Abilities)