Adventure! already has the Palace of the Mind mesmerist Gift, and characters can draw their companions into their mind palace. Just treat an astral plane scene as a normal scene, but there are no true physical consequences once you leave. At worst, "injuries" suffered simply translate into being stunned or knocked out, and persistent physical conditions (ie. broken leg or severed hand) turn into equivalent mental conditions (ie. psychosomatic limp or loss of motor function in the hand).