Although I really like lapping my Belgians on my DMT 325, I agree that sandpaper would be fine for flattening them. These hones are harder than a Norton, so you only have to lap them occasionally. I lap my blue about every 10 razors or so. The coticule gets lapped even less frequently.

If you hone a lot, some kind of flattening stone is very handy. If not, sandpaper works great.

Josh