a couple things popped into my mind when I first saw it as well, mostly because I have similar ideas ruminating recently. the angle for the slab will be set, it seems the extra rock up top would almost take away from the planting so I just disregarded it, buy maybe not, could be a built in backdorp who knows assuming it would be removed, or carved, or mossed later on, and focused on the planting angle, which is nice. I've watched arthur Joura repot several forest plantings recently. The planting would be fixed, and the angle set, so you could swap the rock with another rather easily, given the same relative surface area and planting angle, later on. Or you could rearrange the trees altogether if you wanted. which brings me to my next thought, Beech are a finicky, somewhat high maintenance, needy tree. collecting them straight from the woods,(they only grow in deep, old woods it seems, maybe they grow on rocks elsewhere idk) to a bonsai planting is hardly a sure thing. while I know Giga keeps several beech, im sure they will all live, but its very possible at least one or more may not make it, which would alter the overall planting. it wont probably wont be established for several years, and beech grow slow it seems, requiring one to add or rearrange a tree or two over the next few seasons, providing ample time to find a better suited planting host, if one was dissatisfied.
Ive found that the giant round run off water catcher/humidity trays they sell at the hydro stores may provide an excellent cheap initial forest planting host, that will easily be swapped out at first repot with a bonsai pot or slab. those were my thoughts.