Well, yes and no. Bump from textures is not a real bump, it is an optical effect. So, no you can't make shadows that trace out an optical effect ... not in Bryce anyway. You could put them in by postwork. The only other way around the problem is to create a terrain that incorporates the bump. For example ... a wall made of a primitive with a bump applied is just a perfectly flat object ... no matter how bumpy the bump looks. But, if the wall is created by modeling a terrain into a cuboid shape, and that wall has a grayscale bump applied to it in the terrain editor, then Bryce sees that as a real bump and the light should affect it accordingly. Caveat, however, the more actual "bumps" you put into any one scene, the more polygons you have, and the longer your load and save times will be. (The render time itself isn't governed so much by the number of polygons as it is by the optics involved ...) If you look in my gallery at "So Much for a Quiet Saturday Afternoon", you'll see what I mean. In the outline of the towers, you can see that the "bump" is real. If the towers were modeled from primitives, it would just appear flat.

Could be worse, could be raining.