perhaps make max executive pay+benefits proportional to the lowest full-time salary+benefits with a maximum ratio? The spirit of such legislation would be that executives should then seek to increase salaries of their workers in order to then increase their own.
Sure, but when you outsource, you lose efficiency to an extent. Corporations are large because of advantages they get from centralisation resulting in administrative efficiencies and this would be forcing them to undo some of those advantages.
Even worse executives would ex-insource. There are plenty of permatemp agencies that can take employees off of your payroll, while keeping them still working inside your centralized structure. And there are plenty of corporations who have historically given slightly higher level employees a pink slip simultaneous with an on-site consultant contract.