Beetles tend to rearrange their enclosures as they dig and root around, so anything placed inside as decoration would have to be firmly secured to the enclosure itself before substrate is added. Rooted plants are unlikely to survive the constant uprooting and moss will easily be torn off of any surface. The best you can do is set up something with a bunch of nice pieces of wood and some leaf litter. Decorations will make it difficult to observe the beetles themselves, of course.