Advertise Mag reading when button pushed

There isn't, but try:

setWatch(function() {
  var m = Puck.mag();
  NRF.setAdvertising({
    0x1809 : [m.x>>8,m.x,m.y>>8,m.y,m.z>>8,m.z]
  });
}, BTN, {repeat:true,debounce:50,edge:"rising"});

That should pack the magnetometer data into 6 bytes that you could then read. Hope that helps!