Hate to burst your bubble but, I've got evidence to prove your hardtop theory wrong. You were close, but you didn't look closely enough.
The red lines trace the gaps in the trunk. The gaps are in the same place as the current 6er. Look between the red lines. The green line indicates the perfect seal between the roof and windows.
The gap is still visible here also.
Edit: ALTHOUGH...I have heard the hardtop is possible, I don't think this mule carries one.