Spawners: 3 dimensional spawning

Hi,
have a simple question. Is possible to set spawner to spawn mobs into cube, not just square. If I understand it correctly SpawningRadiusY parametr is simply offset.
Reason is simple, if i want to populate rugged area i have to use multiple spawners with smaller area to avoi mobs spawning in air, or not spawning because of slope.

If it is not possible. Is feature of this kind planned for future version?