well what i mean is that non enterable part of map still contain objects (cars, walls, trees, rocks etc) which can be used to bounce grenades back to enterable part of map ...
maybe it would be possible when dynamic generator is back (to reuse it to generate map border areas)
On prebult mission maps, using the actorclips, this is possible. Undesireable, but possible. Square footage adds the most size to a map from my experiments so far. Adding a, say, 4 step border around the map that's uneterable still allows for bouncing grenades, adds size, and not much versatility.
Besides that, It would drive me batty if I could see perfectly good cover I'm not allowed to get to because of 'game design'.
The best bet is for designers to 'wall' the map, fences, hedges, whatnot, or to have the system detect a grenade leaving the map and it going level with the last piece of ground or just going away.