Thanks guys!!
Yes, you need a lot of patience to do things like that.
In the meantime I tried to be even more patient. I found a nice free soccer stadium model (I think it was Cadnav as far as I remember...)
Actually there's only the outer wall and the towers that are original. I replaced the whole interior, textured it and created an invisible copy of the area where the seats are to populate the spectators on. Four sets of fifteen populations each with a lot of edited instances.
The whole scene is only lit by the luminosity of the lamps, which looks quite good to my taste, but the GI flicker is terrible. Even though I made heavy use of GI caching.
At least rendertimes were a lot faster than with real lightsources in the scene.
Creating scenes like this is very tiring, so there are no players or referees in the scene. Maybe in ten years....