Hi! I may be a little late in replying, but I came up with an idea while reading this thread..
I dunno if you all would like it, but I think the idea itself is "cool" and I may even fix it in my car even though I ALWAYS fasten my seat belt while driving..
So the idea is..well, if you can try recording a message in a device that may say, "Welcome! Please put on your seat belts and enjoy your drive!" or any customized message..
Now the tricky part is finding such a "device" that plays back a recording..We can then connect this device to the ACC terminal or the Cigarette Lighter terminal, so that as soon as the key is inserted and rotated, the device powers up and plays the recording..There! The theory part is now over..Now begins the hunt for such a "device".....