Need object communication as far as 500meters.

Why wouldn't you just link the two objects you need to communicate? If this is a newbie question... alas, I am a newbie to LSL.

Because without the use of a complex script, there is a distance limit on linking.

You should be able to use the remoteaccesspin script I posted.
That will accomplish what you want.
With a sensor in the skybox checking for AV's every 10 seconds, it can send a script to the prim on the ground. The script can set the text or llSay that the skybox is occupied while it is in the base prim. It can be passed a parameter to know which message it should be saying.
I will be in game tonight. IM me if you would like help with it. I have an idea for how the script will work and it is pretty easy.
I don't charge...though you can donate if you like

Just contact me and I will help you.
It's easier to explain once you have the code in front of you.

I just want to say thanks to everyone for helping. I decided to go with the “Shout” with repeaters method. In the end it only added 3 prims to everything. Also we put up 2 status displays for showing the Skybox status. Implementing a second display panel was a direct copy with no changes to the code required anywhere. If I had gone with the remoteaccesspin script method It would have been necessary to add more code to support a second display panel.

You can see this setup at the Island of Rainbow Tiger in front of the Gym entrance.