Unity uses an object called LightingDataAsset to store its lightmaps. Regrettably unity does not expose the data in this object.
Ref: https://docs.unity3d.com/ScriptReference/LightingDataAsset.html https://docs.unity3d.com/Manual/LightmapSnapshot.html

In order to make them work in play mode you ...

I'm playing around with the lightmapper, but when I press play, all lightmaps are cleared. Is this correct beta behaviour or am I missing something?