I've been trying to do the same thing or something similar for most of the day. I threw together a land navigation mostly for training purposes. The goal of the scenario was that your team spawns in random locations (on hard difficulty so not markers on the map) with flares, and compasses. In order to "win" the scenario you need to get back to base. You could either all walk back or if you found one another you could call for a heli extraction. I handled the finding eachother with some cascading triggers and some logic statements and was able to get a heli called up on the radio but landing became an issue. Here's what I ended up with after reading your script and messing with a few things...
Put the following into the Init section of your transport trigger...
LZ = "Land_helipadempty_F" createVehicle getPos Player;
Then if you really want you can put a deletion parameter in once you take off. This line and this line alone creates an empty helipad where the player is standing when the trigger is set off. However it's on the player to make sure that's a clear area. The cool thing about this method is that you can actually waypoint a chopper to a player directly and that waypoint is permanently attached. So no matter where you are on the map the chopper will come get you.
Note that I prefer to do my extractions and insertions with persistent vehicles.
