functional hypocholesterolemia due to very high HDL or low LDL?
Upper bound of HDL doesn't really exist. The greeks are known for their incredibly high levels of HDL and the so called mediterranean diet/genetics combination that causes this is only beneficial to health (supposedly - although never been proven that incredibly high HDL levels are beneficial, they're just not harmful). 

As for too low LDL levels/cholesterol levels - that's interesting. Unfortunately it isn't possible because any mutations that caused this would be fatal, cholesterol makes up an essential part of cell membranes etc. That's why our body makes our own. A mutation preventing this would not even manifest.