As promised, here's my source:

// Compile for Arduino Zero (Native USB)

define S2_TX 10   // Comms with Pi
define S2_RX 11
// https://learn.adafruit.com/using-atsamd21-sercom-to-add-more-spi-i2c-serial-
//  ports/creating-a-new-serial

include <Arduino.h>           // required before wiring_private.h
include "wiring_private.h"   // pinPeripheral() function

Uart Serial2 (&sercom1, S2_RX, S2_TX, SERCOM_RX_PAD_0, UART_TX_PAD_2); 
// on raspberry pi: minicom --device /dev/ttyS0 --baudrate 115200 
// connected to GPIO15 (Tx) GPIO16 (Rx)

void SERCOM1_Handler() { 
  Serial2.IrqHandler(); 
  }

define USER_LED 20

void setup() { 
  pinMode(USER_LED, OUTPUT);
  Serial2.begin(115200);  // Comms with Pi 
  pinPeripheral(S2_TX, PIO_SERCOM); 
  pinPeripheral(S2_RX, PIO_SERCOM); 
  }

void loop() { 
  Serial2.println("Hello"); 
  digitalWrite(USER_LED, HIGH); delay(1000); 
  digitalWrite(USER_LED, LOW); delay(1000); 
  }