Yeah the fact that air can’t even reach the filament inside the spool occurred to me early on too.
Maybe we need @MZip to engineer us an uber drying contraption which gradually respools the filament while it’s being dried so every bit gets hit with some hot, dry air. 
If you search the forums there’s a giant thread he started where he’s forcing dryer-than-ambient air into a dryer using an aquarium pump. Maybe that would help. I might try doing it with the S4 I have, if I get to a point where I feel I need it. In the middle of a nice dry-ish winter right now.
Found it: