The only reason that I can think of for using cyanoacrylate adhesive on an acrylic pen is to fix damage such as bubbles or chip outs. As far as sanding and polishing are concerned, both CA and the acrylic pen material seem to polish out about the same so I can't see a reason for applying CA as a top coat. Besides that, there is a possibility that the CA could damage the acrylic by softening it. No all pen blank plastics are the same. Some might be fine with a top layer of CA if that is important to you. Sometimes Inlace and "Cactus Juice" are used to encapsulate materials that might occasionally have small air bubbles if the vacuum process wasn't done well enough. I think that it is OK to apply CA on those plastics, but if I were you, I would verify that before proceeding.