How can I make a geometric portal instead of a teleporting one?

I watched some tutorials on making portals but they wouldn’t suit my needs at all because they rig it up as an instant teleportation rather than a geometric transition. The effect I want is a player summoning objects from the depths of hell from wherever he stands, and they rise slowly from below. This would not work with teleportation because upon setting actor location and rising form there, it would still hide most of the object behind the projection plane. Anyone know how to do this?

a player summoning objects from the
depths of hell from wherever he
stands, and they rise slowly from
below

Scrub through this and see what it does. Perhaps you could use it to see through the surface. The rest is a matter of moving the object and beautifying the whole thing.

To clarify: you place your portal thingamajig under the floor and use the 3d sphere mask to see it.