I've done that with a few TBP missions, since you need to use jumpgates, but they are stationary so the make-it-in-front-of-the-player issue is moot. It may be possible (as was stated) with using variables, but it would require use of get-object-x/y/z to determine the player's position and then calculate how far away, but the problem becomes putting the portal directly in the player's flightpath. Here's a trick I encountered that Den5 used for his original TBP "In the Beginning" campaign. He would have a ship warp in at a specific location, then use ship-vanish to cause that ship to disappear immediately, but the vortex would still play. So, have your dummy ship warp in in front of the player, using the "in front of ship" option in the arrival cues, and give the player's ship as its arrival anchor. Then, you need events to both cause the arriving ship to vanish, and another event to get the ship's x/y/z location. Then, base your end-mission cues off of that location. If you can't base your distance-from-this-location check of this kind of location, then a create-ship sexp could place a stealthy and invisible-textured object at that location, and have the distance-check for end-mission based off that.
BTW, you also need to make sure that the player's ship is never-warp flagged.
