While this is useful, I don't think it would completely resolve the noted "tons of send syscalls" issue. QUIC performs it's own flow control and I don't think it can just send all the packers composing a file at once (all the time, at least)
If your server handles many connections simultanously you can still bundle a lot of packets in a single sendmmsg syscall, it can dispatch to a different destination address for each packet.
sendmmsg (or the upcoming io_uring) let you send multiple UDP packets with a single syscall.