The reason I thought of just tilting that rock over towards front right is it will make nice little overhang and have that cool hole in it and led light will look cool shing through the hole.
Wood. Remember that wood has a capillary network, itís how it pumped water and nutrients to its leaves. In a dry piece of wood those networks are now filled with air. If you completely submerge a dry piece of wood the outer layer become permeated with water which has a higher viscosity than air and completely seals the air in the wood, only way air can get out is osmosis through both the water and wood fiber. It can take months if you do that.
To water log a piece of wood start with small end 1/3-1/2 in water and always leave cut end (capillary pipes) completely dry so air trapped in there has a easier pathway out of wood, then raise water every couple days till itís submeged or close to. Then if needed drill a hole and zip tie a ballast stone or for odd shapes just epoxy a stone on.
For seeding rock with moss my fav simple way with flattish areas like back area of that rock probably is (?) is Sandwich bag plastic . Take piece cut slightly bigger than area you want to start, make it big enough so drapes over edges about 1Ē then roll over edges then take lead plant weights pieces and poke them through plastic around edges. Youíve made a weighted clear plastic tarp, cut you some 1/4Ē slits or X abouts every 1.5Ē so it can breath.
Draped over back of rock folded in half and take angle planting tweezers and just start tucking moss under wrap, as you fill it up pull wrap forward to expand area. When all filled with starters if rock has odd shaped dimples/curves just set a small pebble/plant weight to push wrap down so moss makes good contact with stone, is perfect for doing this ,itís flexible, itís clear so light get through, youíve cut some slits so it breaths, moss wonít root to plastic wrap but it will root right on to that calcium based stone. Wait 3+ weeks, pull tarp off, done.
If you do get some driftwood coming up from back end of rock like I suggested, that little intersection where calcium laden rock meets wood, meets iron laden gravel would be nirvana for nice crown type plant like sword or dwarf lily. Java fern and Anubias would do great mounted right there in crevice. 2-3 crowns of crypt undulata red would make a nice centerpiece plant there, makes a nice 7Ē reddish vase shaped clump.