Thanks for the update!
I share the sponza test file: https://drive.google.com/file/d/1bWUxwoHGBrepbyATCLLj7xBapDaHaAmH/view?usp=sharing
I tried to have only the ground to reflect using ImageRender and upbge 2D filters API but we can see (a bit) the sky reflected too in other parts of the scene.
Anyway, this gives nice results:
After, to have a real roughness map or a specular map, maybe it is possible with Render Attachments patch from the main upbge developer in 0.2.4. It might also be possible using some hacks with ImageRender I guess.