Ah, now that I'm more awake, I see the problem- the write is unsuccessful because I haven't yet implemented grow-on-write. None of the consumers that I had found relied on it, instead choosing to ftruncate() it to the size they need before operating on it.

I think the best course of action might be to disable the test on FreeBSD for now (unless you're ok with tossing in a truncate to expand the file), because it might take me a little bit to get around to this one.