Since you are going with a "natural " look, litter and all I would think that marbles would not fit the theme. I also agree that fish waste would settle in the spaces between the marbles and both be unsightly and require more upkeep not less. A moss bed might work and could partially hide some of the searchable items
I think you may have hit upon a major item - the theme is "Natural", and the lake does not have a natural bed of marbles. Sometimes you have to start discussing things in the open to realize you need to dope-slap yourself and say "what was I thinking."
OK, my original premise was that the coral substrate adds a burden on cleaning - not so much during the season, but, at the end of the season, we really try to get the substrate clean before we put it away for the winter. All those hollow shells are pockets for dirt/mold/algae/whatever and we spend about 2 weeks trying to get them clean and dry. The marbles concept was to get the end of season cleaning streamlined. There may be a better substrate option that is aesthetically pleasing and has minimal clean-up at the end of the season.
I'm not familiar with the moss bed concept - is that a live plant scenario? Would it be feasible for a 4 month aquarium lifespan each year?