Short answer: you can’t flag submaps that way.
Long answer:
The mapbuffer is used to store/load submaps. It does not process them in anyway.
The “reality bubble” is the map. Anything that is contained in game::m. This object contains a grid of “active” submaps (see map::grid). The map class itself acts as a wrapper that uses the specific submaps. For example to get what terrain at (40,77) is, the map translates that into the submaps coordinates (40/SEEX,77/SEEY), gets that submap from the grid and looks up the terrain at (40%SEEX,77%SEEY) in the terran array of the submap.
Active items are processed by map::process_active_items, which is called somewhere in game.cpp each turn. The function looks through each “active” submap and each item there and processes it.
Note that NPCs are different. They are stored in the overmap and only a pointer to nearest npcs is put into game::active_npc.