It is possible. Even without any closed doors or sealing the spawning area until the lift is fully on top of the ground. Haven't you played HL2 through with commentary on? They did the same thing with the barney's crowbar and the bridge between two buildings. Do the lift or whatever is needed, place prop_static on top of the lift which has the vehicle model and apply no collision with brushes to it (no collision with displacement isn't needed because displacement can be walked through from the "wrong side") and parent it to the lift. When the lift has reached the position it needs to, exactly at the same time remove the prop_static and spawn working vehicle model to its place. Tada. There we have a lift which lifts vehicles from underground and works. Only one small harder part in this is the way how to cause the model make displacement in the hole area transparent so it looks like the vehicle rises from a long tube but I think even that is possible because removing the polygons of displacement is possible, just that its collision mesh doesn't update but we don't need that. And when the vehicle factory is destroyed it changes the transparency setting from 255 to 0 again and the displacement is visible again.
You're forgetting that many maps have nodraw brushes right underneath the displacements, so you couldn't really put anything there, since it would be outside the map.
the awkward vehicle pop Faunze-- if your primary concern is to prevent the player from seeing vehicles 'pop' as they do, why bother with this complicated elevator scheme when you could just have the area be covered until the vehicle has spawned? As far as I understand it you could just keep the control panel outside of a garage/enclosed area that spawns the vehicle. When it's ready the big door opens, the vehicle automatically slides out, and then the door closes again. As far as the player is concerned the vehicles could be delivered by storks-- it wouldn't matter because you've eliminated their line-of-sight with the spawn. Also, if you're doing this for the NF it would seem to fit in better than the elevator. It's a cool idea tho. The only problem I can imagine is if people try to run into the garage when a vehicle is being ejected. Could you solve this with an invisible wall? I'm not a modeler so I don't know the ins and outs of these sorts of things...
I think it would be possible to have that kind of invisible wall there. This closed vehicle spawn would also solve two problems. -People not driving out of the vehicle factory and blocking the vehicle spawn. Now the vehicle would be pushed out of the vehicle factory before player could control it to ensure proper working of the doors. -Enemy team driving in the vehicle factory and blocking the vehicle spawn. Now you couldn't drive in the spawn anymore because the door is blocking your way. And how I see the door is that it would be similar door to old western saloon doors. Like this one: http://www.salooncafedoors.com/images/Cafe_door_Iron_accent_saloon_doors_large.jpg That door would prevent the way out of the vehicle spawn being blocked by enemy vehicles, other vehicles, walls, turrets or any other obstacles (add instant death kill zone on those doors when they are moving.) That way the vehicle spawn wouldn't jam because of stuck vehicle which can't get out of the spawn.
Just have the doors open and close, the vehicles are pushed out at such a speed that they can't stay in there anyway (and if they can, increase the push). And make it impossible for infantry (and preferably tanks) to get back in
