I was under the impression there already is a requirement similar to this yet the general tactic is to hire someone for a job far, far under what they are actually going to be doing. For example, hire someone with a PhD and years of experience to be a "web designer" (a very highly paid one, making 70k!) on paper but who actually does complex distributed systems that happen to serve web pages.

Honestly I'd favor an absolute minimum limit. Hiring someone through this kind of visa should be an extreme method you use to find talent you simply cannot find here at any price (not sidestep the American job market and devalue labor) since their visa is contingent upon them willing to work for you -- it's nearly indentured servitude. So you should have no problem paying them at least $200,000/year.