I am glad that you clarified what you are thinking. At least as you are describing it, (i.e. "Reserve buoyancy is a hulls ability to carry a load over and above displacement".) that is not in keeping with the way yacht and ship designers use the term, "Reserve buoyancy". What you are describing rarely if ever is considered as a part of yacht design.
"Reserve of Buoyancy" is a term that come from time begin of yacht design. Sometimes shortened to reserve buoyancy. There are a number of chapters in this subject and the importance of reserve of buoyancy in yacht design. Here is a quote from page 12 of "Naval Architecture" printed 1877.
".. commonly used to express the volume and corresponding buoyancy of the part of the ship not immersed, but which may be made water tight.."
"The under water, or immersed, part of a ship contributes to the buoyancy; the out-of-water part the reserve of buoyancy, and the ratio between the two has a most important influence upon the safety of the ship against foundering at sea."
It sounds like Sea Hunter described it correctly.