I'd be interested to know more myself, and may have to know a bit more for the sake of the litriture review. I cant put my hand on Wangals book at the moment, but think he gives some history. Given that Tibetan buddhism is Buddhism apadpting to the background of the Bon religon, its fair to say its fairly old, as Buddhism made its way into Tibet around 650 CE. In terms of dream yoga and buddhism, I've never heard of it in relation to Zen or Theravaden Buddhism. I think it is a Tibetan thing. They say the 1st Karmapa attained his enlightenment in a dream (I think..), and currently Dream yoga seems more spoken of in the Kagyu tradition of tibetan buddism. Looking in Charlie Morleys book now, the forward by Lama Yeshe says, "in the tibetan tradition, the refined discipline of Dream Yoga has been practised for a thousand years, and as one of the famous Six Yogas of Naropa it is an important element in our intensive for year retreats at Samye Ling Monastery. The stability of the lucid dream is the foundation of Dream Yoga"..
If/When I come accross more I'll let you know...