I just sand with 100 grit down through the rind (cambrium layer). When it changes color, I stop. I only do the nodes lightly until the rough stuff is gone. I don't think I would use any type of sharp edged scraper. One careless move and the fibers could be violated/cut. And the stuff on the top takes the most tension. Given the amount of work it takes to prep and glue-up the boo, I wouldn't risk a careless mistake. Sanding the boo is my last step before finishing, so I have a lot of time invested when I get to that point.
Ken,
PS I have seen bows where stain/stencils were done on the boo, but they only sanded the hard outer portion of the rind off and left the (creamy colored) middle rind in place. This part of the bamboo will hold stain, unlike the main part, which won't take stain easily.