I know two people who I have learned a lot from that meditate in the astral. One of them loves japa and he said he likes to do it there, while his body is sleeping at night. They do other techniques as well. Most energetic practices and meditations have nothing to do with the physical body, and so if one is advanced enough in astral projecting and meditation then there is no reason not to be able to. Also, there are temples in the astral that teach higher meditations and teachings I hear.