I agree, hiring managers aren't empowered. Any company I've worked for, you just need the person signing off on your expense reports to "approve" any out-of-policy expenses. So, you have to get your manager on board. Experience has been that business unit managers know where the business is generated and won't put it at risk for the sake of not angering the T&E folks in the back office.
There is actually no way to buy a C or F ticket in my company... the booking software won't let you. If you call the company travel agent with an out-of-policy booking, the CFO's office will call asking for an explanation (the travel agent will not book the ticket, they flag it for "policy review"). And forget about booking outside the corporate travel system, unless you're in the market for a new job...
As a manager, I can approve "out-of-policy" expenses, but those type of things are $52 for a car rental when the limit is $50.