Can an "executive only" HSA contribution plan for executives be offered?

Outside a Section 125 plan, the plan is subject to comparability and this would never be allowed unless the executives were the only employees (the rules of comparability would fail). If an individual were to structure the contributions through a Section 125 plan, the issue becomes a discrimination issue as a plan or employer cannot discriminate against lower paid employees. In other words, higher paid employees (i.e. most executives) cannot be given contributions that lower paid employees don't receive.