Hi Piero,
That looks like an awesome tool! Thank you so much for sharing!
For now, Cesium doesn’t really mess with the shaders of the glTFs that come in, unless the model is using the KHR_materials_common extension in which case Cesium must generate one. So the answer would probably be to provide a text box to edit the shader, then re-load the glTF. Support for more scene lights is currently on our long term road map.
Keep making awesome graphics tools! It's cool demos and tools like these which really make Cesium shine as a platform. =)
Thanks,
- Rachel