Capping executive salaries is a good idea in theory, and I've put some thought into it. From where I'm standing, the only way you could do it fairly would be to make executive salaries only a certain percentage higher than their lowest paid full time employee. That percentage would be the issue, but it would certainly limit the amount multi-national corporations could pay their execs, but still allow them to pay the sort of money which is usually comensurate with the knowledge and experience required to fill such positions.
eta: This system would obviously very likely improve the salaries paid to lower income earners which could be a good thing. Kind of like re-distributing the wealth.