Hi, I have done this many many times. The easiest way to make sure the knots do not get screwed up is put a layer of heavy duty styling gel on the upside down part of the base. It does not seep through to the hair and protects the knots. After the dye job is finished rinse off the gel with warm water. Piece of cake. I have done this with almost every blond piece I got to dye them dark brown or blond with brown highlites (reverse blond highlights I guess)
Good luck
Michael