Yes, my understanding is that I should be able to emulate sendfile via splice. The problem with that is that splice requires one end to be a pipe.
So I think this means two extra file descriptors per connection (one per side of the pipe). And per connection this adds 5 slots in the submit/completion queue, with a LINK dependency. Maybe the trade off is worth it. I've not done concrete experiments with it, but I'm guessing it would be if the saved copy_from_user is large enough.
So for optimal performance this may mean using write() for short files, and a pipe(), a pair of splice() calls, and a pair of close() calls, for larger files.
Edit: I guess I could save some ops by reusing pipes, but then I'd have to make sure to flush them. Would add some complexity.