As a random QA guy, I must mention that the cost effectiveness of determining whether a website is usable when high is questionable at best unless we are talking about a snack or pizza ordering website.
You're much more likely to have a visitor base that is likely to be intoxicated, and Browsing Under the Influence is still legal. I would thus offer my services as fellow QA to determine website usability while under the heavily intoxicated; client may specify desired level of user intoxication from mildly tipsy to fall over drunk.