It looks like my memory may be faulty. After digging in the code, I came up with nothing. And then I remembered this port: Light Control Fixes and GLTF Punctual Light and Unlit Material Extensions - #14 by sgold
In the loader, the material is created here (jmonkeyengine/GltfLoader.java at 153e3e8370876feb420f4a727441ca9c04836fe4 · jMonkeyEngine/jmonkeyengine · GitHub) if one it not specified, but it looks like there is no way to specify it. Perhaps a GltfModelKey could be created in the future for providing some settings on loading gltf modes. But for now it does not look possible.