How to serialize an Ext sequence in rmp_serde?

Question

I'm trying to modify ormsgpack so that it will serialize the extension types I need, starting with distinguishing tuples from lists: tuples are extension type 0.

This works:

impl Serialize for TupleSerializer {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        let mut buf = std::io::BufWriter::new(vec![]);
        let mut ser = rmp_serde::Serializer::new(&mut buf);
        let len = ffi!(PyTuple_GET_SIZE(self.ptr)) as usize;
        let mut seq = ser.serialize_seq(Some(len)).unwrap();
        if len > 0 {
            for i in 0..=len - 1 {
                let elem = nonnull!(ffi!(PyTuple_GET_ITEM(self.ptr, i as isize)));
                seq.serialize_element(&PyObjectSerializer::new(
                    elem.as_ptr(),
                    self.opts,
                    self.default_calls,
                    self.recursion + 1,
                    self.default,
                ))
                .unwrap();
            }
        }
        let _ = seq.end();
        serializer.serialize_newtype_struct(
            rmp_serde::MSGPACK_EXT_STRUCT_NAME,
            &(0i8, ByteBuf::from(buf.buffer())),
        )
    }
}

But it performs badly, because it's copying stuff in and out of that buffer.

I want to just write a couple bytes, 0xd6 and 0x00, before serializing a sequence the normal way, like for lists. How do I do this?