Umberto, thank you for your reply.
I have tried your workaround as described. I've assigned my custom liveries to remote stands that won't ever see use & then added the codes to the "Airline Codes" field as such: "SAS GROUND HANDLING, MENZIES AVIATION, AVIATOR". Unfortunately it does not work. GSX simply choses "Jet Aviation" which I'm assuming is the global default.
Even when I attempt using the default textures provided (MENZ, AVIATOR) this behaviour repeats.
Any thoughts on this or what I might be doing wrong?
Finally I must admit that I don't agree with your logic that if a custom livery is present, no other livery should be allowed. The fact that I create a custom livery doesn't have any influence on whether or not multiple ground handling agencies operate on the same stands. It simply means that there's an operator missing or with inaccurate textures at my local airport, which I would like to rectify.
Would you consider changing the way this works in GSX?
Thank you in advance.