With regards to using a code you are not eligible for, I spoke to an Avis agent here in Vancouver and he said the biggest problem was getting insurance. Most of the corporate codes now have a pre-negotiated insurance policy that isn’t part of the rental and now is held entirely by the corporation. You would not be able to add any third-party insurance to these reservations. Credit card loss damage waivers will not cover you.
So if an accident where to occur it sounds like the individual would be 100% liable.
For me the risk is far too great.