What I am using (although not exactly what you asked for, it does keep liveries out of the main game folder):
I have an Entry.lua file in my saved games folder under Mods\aircraft\Liveries Import. I don't remember whether I had to create this file or whether it already existed and I edited it:
The contents of Entry.lua:
The line mount_vfs_liveries_path points to any location on your PC to store liveries. On my PC at that location:
Note that all custom liveries live here, not just for the tomcat. I don't know if there is as way to do a plane specific folder or not.