The best I can ever do is ask for 4pm, and actually check out closer to 5 without penalty.
Busy properties will give you the half day speech at the drop of a hat.
With properties such as the San Fransisco Hilton, I no longer even try. I tell them I'm going to shower and change after my work day before my flight, charge whatever you want. They usually bill me $160 to stay till 6pm - but I don't feel like a beggar.
This has always been an inconsistent benefit for VIP's, having much more to do with who you speak to than any policy.
As a suggestion, rather than calling the front desk, if you were left a personalized letter/note in your room welcoming you to the proiperty, call whoever signed it. They always ask to be contacted if anything can be done to help you.