Arma 3 exposes exactly this capability through a single scripting command:
setTerrainHeight [[[x, y, z], ...], true];
The community mod Deformer by Sams (Steam Workshop ID 2822758266, https://steamcommunity.com/sharedfiles/filedetails/?id=2822758266) is built entirely on top of that one command. It provides raise / lower / smooth / flatten brushes in the 3DEN editor, works on any terrain, persists edits into the mission file, and is SP and MP compatible — in multiplayer only the server needs the mod, because the engine replicates the height changes itself.
Architecturally the mod is thin. Everything except the actual write is ordinary script: a brush weight table, a hash map of [gridX, gridY] → [newHeight, baseHeight], an undo stack, and a compressed serialisation of the edit set. The engine call itself appears in only a handful of places, batched at ~10,000 points per call. The one thing the mod has to work around in script is static object placement — it disables simulation on affected objects, applies the heights, restores world positions, and then patches up client-local and simple objects over the network. That workaround is precisely what an adjustObjects parameter should make unnecessary.
The equivalent mod cannot exist in DayZ, and the only missing piece is the engine call.
Why this looks feasible in DayZ
The engine already implements heightfield editing — it is simply not reachable from game script.
1_core/workbenchapi.c exposes WorldEditorAPI.BeginTerrainAction() / EndTerrainAction(), i.e. undo-transaction brackets around terrain edits, but no vertex setter alongside them.
5_mission/gui/scriptconsole.c lists the Buldozer input bindings, which include UABuldSampleTerrainHeight, UABuldSetTerrainHeight, UABuldElevateMod, UABuldSmoothMod, UABuldFlattenMod, plus the full brush control set (UABuldBrushOuterUp/Down, UABuldBrushStrengthUp/Down, UABuldBrushRatioUp/Down).
So sampling, setting, elevating, smoothing and flattening terrain with a configurable brush all exist at engine level — the same operation set Deformer offers in Arma 3. The request is to surface that capability to scripts.