There's alternatives, but my actual requirement was to verify whether 'filesystem locking' works on NFS. Which is really vague, I know. But that was why I was seeking a system call based option, as that gives me a basis to say "it works" in at least an approximately language agnostic way.

FWIW - on a 2.6.12+ Linux kernel, both flock and fcntl work. You will still have platform specific issues doing it regardless, but that isn't a new storage when it comes to NFS.