I love mopani wood. I would put the gnarly side out, personally. It has a lot of character. I've both superglued/tied plants on wood or stuck them in holes. Both ways worked. You might want to try different methods for different plants.
Attaching to either side will work. I like putting smaller Anubias in the nooks and crannies where they'll fit. If you have any large anubias plants with long thick rhizomes, I prefer to put those on the back side and glue or tie them in a way that shows the leaf and stem but hides most of the rhizome. Then the roots can creep down behind the wood without covering up the cool intricacies....will the plants grow better on the smooth side?...