For now, you can set eyeOffset on the label with a negative z value:
label: {
text: "A",
font: '14pt monospace',
style: Cesium.LabelStyle.FILL_AND_OUTLINE,
outlineWidth: 2,
eyeOffset: new Cesium.Cartesian3(0.0,0.0, -1.0) // <----------------- negative z value so it appears behind the points
},