We're not going to add one because it's already there.
At http://os.inf.tu-dresden.de/fiasco/ux/using.shtml there is some text saying:
n tasknumber Allows the specified task number to perform native system calls and call directly into the Linux host kernel using int 0x80. This is useful for hybrid Linux/L4 programs.
If you think that is not good enough, you are of course welcome to suggest a better description.
I noticed the description and I knew "fiasco -h" also shows it, but what I meaned is that it may be difficult for a freshman to associate it with con-ux. Especially, that the example in the page with dope-ux does not need this option.
Of course, it may be just me that can not understand :-).
Regards,
Wei