i have had a thought on this one, not very good but the best i could think of.

you could do other things to make the player stick to the sides of the sphere instead of with gravity. you could put loads of ladders round the edges, and on the bottom you could make loads of triggers to target_push (like for jump pads) that push the player up to the bottom of the sphere.

yeah, but once they moved to the bottom the player wouldn't be turned upside down, so therefore he would be walking on his head......