Hello Alex, thanks for your reply.
I had this problem in H21.0.368 (production build), but I just checked and I don't see it anymore in 22.0.406 daily build. That is, if I drop a fresh instance of Bake Geometry Textures COP (Cop/bakegeometrytextures::2.0), because the old instance of this operator, which I have in my scene, still fails to compute UV boundary filling for custom attributes - at least for Cd. So, I believe you guys have already fixed it.