Since you build your packages with 7-zip under Windows, permissions should not be the problem, but you can check the permissions with the 7-zip filemanager, or on command line:
Code:
7z.exe l -slt KoboRoot.tar
(This is a lowercase L after 7z.exe)
But I think it is more likely that the content of your package breaks the boot. One likely cause is Windows style end of line format, the other is syntax errors in scripts, especially in rcS.