> The kernel is being built as 'a.out' native (if
> I understand that OBJECT_FMT=aout accomplishes this).

That didn't answer the question I asked.

Does your kernel config include the COMPAT_AOUT option?  If so, and
the userspace binary issing the new system call is an a.out binary
rather than an ELF binary, it's using a different syscall table so
that certain emulation hacks can be done to allow the old a.out shared
library loader to use the "same" pathnames as the new elf shared
library loader.

Add your syscall to sys/compat/aout/syscalls.master as well (and run
make in that directory, etc., etc.).

The cleanest thing you can do at this point is to upgrade to a
complete -current snapshot (userspace and kernel) rather than trying
to use a -current kernel with an older userland.

					- Bill