Okay, here's a silly question for you. What conceivable use could a Zorro slot on the bottom of the motherboard have had? 
http://www.ebay.com/itm/Amiga-2000-Prototype-Pre-Production-Unreleased-Rev-4-0-WORKING-Commodore-/222020409461
Guesses in order of perceived likelyhood:
1. debugging. Either because of the form factor, or because Dave made the same mistake when he did the first prototype 68030 card and routed the pins the wrong way round. It might have been cheaper to move the slot than rewire the test harness.
2. there was a daughter board that could be attached to it that added more zorro cards, so they could test that boards would work if they ever had more than 5 slots. It would probably be easier to plug something like that in from the bottom.
3. you put an a2000 underneath with a riser that connected the two a2000's zorro bus together for testing with two cpu's driving the bus. This would probably involve some hacking to autoconfig to be in any way sane.
