Right. Technically, you could cover the entire chip with TopMetal (no upper density limit?) and open up almost the entire inside area. However, this would be pretty useless since you would then have TopMetal everywhere. Even then, I don't think it would be accepted for manufacture. The rules seem targeted at "normal" bonding applications.
So I guess it's fair to say the min. pad opening is 40x40um, and there's no way around this by opening up the whole chip.