A Global 5000 requires two pilots who are both QUALIFIED to fly it. Meaning at least a Commercial Multi-Engine with appropriate type. There would be no way you could sit in the right seat and "build time".
I believe that 91 rule comes out of 91 Sub K, which pertains to fractional flying.
Insurance companies rule aviation. It doesn't matter if the boss of the company wants you flying his brand new jet, if their insurance says "no" because of your qualifications then you're stuck. Unless your boss wants to pay premiums up the butt to insure you then I guess that's a way.