I think you'll find it is. In Sven Co-op, there's a map called stadium3. That allows you to put most monsters (Including apaches and new Sven monsters/monsters nicked from opfor), but there's a limit to how many you can have at a time...and Sven's monstermakers have more options than HL ones. It also has an airstrike table, a flooding button, a light switch, and a massive airstrike that will destroy everything. The map doesn't have stands, it's just one massive box with a sky, an arena, and teleports which allow you into the arena.
Umm, doesn't this contradict itself? You can do it but it's crap?