FYI you aren't supposed to use an OS-specific header type, since you aren't the OS. No one can stop you, of course, and since there isn't an application-reserved range, you had to pick someone's reserved range.
Says who? You make it sound like there's some authority out there assigning these numbers. It's not like GNU had to go out there and beg them for it. They just did it, and the world just had to cope.
Also, I'm building lone directly on top of the Linux kernel. That absolutely makes me the "operating system", in the GNU and POSIX sense. Lone/Linux, if you will. The idea is to take this far enough to boot Linux into a custom lisp userspace.