This is presumably due to the MeshInstance it maintains not being present in the editor tree, and thus not being exposed to the BakedLightmap node.
BakedLightmap appears to have a bake() function that takes a Node to bake from, but I'm not sure if that means it'll be able to see a MeshInstance that doesn't live in the editor tree. Need to experiment.
This is presumably due to the MeshInstance it maintains not being present in the editor tree, and thus not being exposed to the BakedLightmap node.
BakedLightmap appears to have a bake() function that takes a Node to bake from, but I'm not sure if that means it'll be able to see a MeshInstance that doesn't live in the editor tree. Need to experiment.