@Trachu: Yeah, you guessed right with the Sprite allocation.
From having a look on it, I guess what you describe is possible, but rather complicated and kind of unflexible.
I am not really in need of more Objects, though, since in addition to the Sprites I still have my merry old BOBs flying around, which only have the problem of being a bit less colourful than the Sprites.
Building such a powerful sprite multiplexer would be an interesting exercise, though...
