I'm copying a shape to points - the color, direction, and pscale are taken from the point for each instance, but transparency via the Alpha attribute is ignored, even though the alpha works visually in the SOP. However, the general alpha slider in the stamp point works. Does this mean the alpha attribute isn't supported, or is some other attribute implied?

Checking the code, we aren't binding Alpha :< So the alpha attribute is not supported. I've submitted an RFE (152678)