I see they say they can't use the ffi [1] as libc is not in the initrd which is kind of unfortunate as it makes it a lot more work. Issue is that glibc is insanely big for an initrd. For my similar project based on LuaJIT [2] I use the ffi and Musl libc [3] which is a sane sub 600k shared library (all in one file). Will put up a bootable system soon.
[1] http://git.savannah.gnu.org/cgit/guix.git/tree/gnu/packages/...
[2] https://github.com/justincormack/ljsyscall
[3] http://www.musl-libc.org/