Good idea. In saikei, many elements can be added to mimic a landscape, they do not all have to be real. I have seen plastic grass used. Colored stone or blue epoxy to mimic rivers and lakes. Some people fill their scene with figurines of animals or fishermen. I have used deadwood and driftwood to look like fallen ancient logs.
Moss hasn't gotten to this one naturally yet, so if it does I'll have the option to trim it or let it go wild. The scene might become a hillside meadow or a jumanji mountain. Moss will probably take over, as it does with many of my other bonsai. The moss ends up being trimmed almost as much as the trees are in early Summer.