The earliest reference to Yoga that I can find in the Watchtower Societies publications is in the February 18, 1942 CONSOLATION Magazine on page 30. Let the Fear Mongering begin! :
"The Yogis of India, in order to make conditions easy for the demons, commence with exercises to stupefy the mind. They hold one object in the mind's eye from periods varying from a few seconds to days, according to adaptability. Then follow intense breathing exercises, and afterward extremely difficult and demonized exercises which twist the human body in terrible convulsive positions; and only a Satanic mind could conceive of thus dishonoring the body Jehovah gave. These postures are held indefinitely; thus showing the entire lack of reason behind it, and complete demoniac influence."