I tend to agree with doug's direction, but Tomh could be correct also. I do suggest that you contact Hy at Peel Away for his comments. He is pretty good about responding.
That 20% moisture reading is higher than typical around here. We tend to run within a per cent or two of 12%. You might check some other wood to see what that runs.
I'm not familiar with working with with cypress, but I would assume that it would dry pretty much like a typical soft wood. You set the pitch to the maximum temperature you expect and if you don't exceed that, the pitch stays put. I wonder if something has reversed the way the pitch is reacting. I just was looking at some cypress in one of the woodworking stores that appeared to be very dry and it was also very light weight--much lighter than Southern Yellow Pine and more like a very well dried fir. Don't know if these comments will lead you toward a direction, but they might be some kind of idea starter.
Since the house in question is in the Deep South, you might suggest checking with the local historical societies or others working with preservation efforts there. Local lore and knowledge can be very helpful sometimes.
I haven't seen any cypress knees around here in a long time--or Spanish moss either. Guess I should get out of the basement more often. LOL