Sure you can go barebottom in a fuge.
Live rock would be important in it, as well as macroalgaes.
The point of a fuge is to have a "refuge" for macro and micro organisms away from predators as well as to grow macroalgae. By eliminating a sandbed, you are not letting those kinds of critters grow, multiply and thrive to clean and filter your water. But that doesn't mean that organisms won't grow on your rocks either.
Sand, especially deep sand beds in refugiums offer a way for nitrates and phosphates to break down into Nitrogen gas naturally, but is not needed directly for denitrification.