Mainly what you see in a Black Widow riser is the pattern made by the many laminations they use. These can be organized in a way that is quite striking. For example, look at the BW website under bows at the picture of the PTF one-piece recurve. So even though something might have a straight grain, you're not really seeing that side of it, like you would with your limb veneer; you're only seeing the edge of the laminations.
For the small pieces on the top and bottom of the riser where you would see the grain, you could probably pick anything you want, even your redwood veneer. The important thing is to make sure that riser is strong, which I'm sure BW take care of, but I don't think that the last little piece on the top and bottom matters that much as far as the integrity of the riser is concerned.