By default, the last scenery installed with the add-on.xml method will be always on top, assuming none of them use a <Layer>XXXX</Layer> command in its add-on.xml, which can alter the priority.
We *COULD* have set an extremely high ( maximum is 9999, I think ) value for our Exclude, but that would be very rude and other developers might have the same "smart" idea of doing that, so I don't think is a correct approach trying to force yourself on the top, which would likely require constantly checking the ordering and eventually change it to be sure you are always on top.
We don't do that, it would be a mess, imagine multiple add-ons all wanting to stay on top and all automatically re-arraning the ordering, all at the same time...
Sceneries added with the add-on.xml method cannot be rearranged with the sim own interface, you'll need the free Lorby Addon Organizer for that, which is the best option until the sim will eventually add this option.
Another thing you can try, is to reinstall GSX, which MIGHT put the Exclude back on top. But again, only assuming no other area used a <Layer> command, because if it has, the Layer value will take priority.
Note that, the FSDT Exclude area doesn't HAVE to be on top, it usually enough to stay on top of all sceneries you would like to replace their standard jetways with GSX L2 versions.
