The TODO entry was about specifically va_list. Now I'm not sure how the "new" stdargs API can be manipulated through CFFI - AFAIK it is purely a C compiler thing - you cannot foreign funcall to va_start and va_copy. The only way I could use it is through lisp implementations that support a form of c-inline, or through "grovelled" code which would be loaded through a shared library.
IIRC libffi has support for dealing with varargs.
there could be a new system called cffi/c2ffi+libffi, or somesuch, that expands to cffi+libffi definitions? or is the cffi + libffi integration transparent and loaded on-demand? looks like that.
or straight out depend on libffi by cffi/c2ffi? i don't think that's a good idea, though, without some hard-pressing reasons.
i don't feel the urge to get into actually hacking on this, but i'll be happy to answer questions about the cffi/c2ffi code.