I’ve got x3 Geberit Omega concealed cisterns converted to pneumatic button use with disguised top access similar to Chris35’s first photo. Went for wall-hung pans for easier cleaning underneath. Same course of tiling throughout bathroom (floors and walls) and the ‘access tile’ held in place by gravity on timber supports underneath and with matt silicone that matches the grout colour (Mapei). Its completely disguised. Should the need ever arise, just cut the silicone and lift out the tile. Clean the old silicone off, pop the tile back in and reseal. Easy. Fortunately, the cisterns and button are still working just fine 7 years on.