I found this program that is supposedly a Level Editor for Starfox, but I have no idea how to use it:

http://pages.infinit.net/voxel/
The download link seems to be broken now, but luckily I downloaded it a long time ago. I've uploaded the Windows version here:
http://www.fileden.com/files/5938/SFXedit093b_Win.zip


I was going to submit it to the utilities section, but since I don't know how to use it at all I don't really think I should be the one submitting a "description" about it. It seems to be a pretty advanced program, and I think you need to understand SNES assembly to actually use it.