I've only tried this with new hulls that I've created.
If I make a custom thumnail for a new hull of mine and place it in the thumbnail folder for my mod, the game will not load it and will create it's own thumbnail from the model itself.
Now, if I place that same thumbnail in the main thumbnail folder for the game itself, it will use it then. It will not however look in the thumbnail folder for the mod itself.
-Stick