Hi David,
Could you try the following script? Replace 
tree.png with any of your transparent textures. When you run the script it should display the texture spinning in front of you. Press the spacebar to toggle between enabling/disabling lighting. At first the texture should be dim, but when you disable lighting it should always be lit. Let me know if this is not the case.
	Code:
	import viz
viz.go()
quad = viz.add(viz.TEXQUAD)
quad.translate(0,1.6,3)
quad.spin(0,1,0,90)
quad.texture(viz.add('tree.png'))
quad.lit = 1
def mykey(key):
	if key == ' ':
		quad.lit = not quad.lit
		if quad.lit:
			quad.enable(viz.LIGHTING)
		else:
			quad.disable(viz.LIGHTING)
viz.callback(viz.KEYBOARD_EVENT,mykey)
 Thanks!