Try checking with an FBO from which you have operated in the past. The lady at the desk of my FBO (the charter company I fly for is part of the FBO) calls the local Enterprise rep. and they in turn set up a rental at my destination at a constant $21.99 per day whether personal or on a trip. They have saved me a ton of cash in the past. We have that deal with them since we use Enterprise exclusively. It may be a compact car that a cyclist could devour, but it is a set of wheels, usually pretty new and it beats walking. If that doesn't work, try an aopa discount number and I think NBAA may have some numbers too. I realize you asked for airline employee discounts, but since most airline pilot's overnights don't usually involve car rental, you may want to explore this option. Good luck,
Kevin