You rang?
Right, I'll give some definitive answers here. The Checkmate board adds a Zorro-II slot, but it's not totally A2000 compatible as it lacks DMA arbitration logic. DMA will still work for one device so long as there's nothing else on the CPU bus that can also do DMA, and that will be 24-bit only so any 32-bit accelerator RAM is out for that DMA anyway. Autoconfig is there and works both on the Zorro-II slot and the A500 passthrough slot, with the Zorro-II slot being first in the chain.
The A500's side port is pretty much identical to the *original* A2000 CPU slot, but the B2000 slot added some extra bus arbitration functionality, e.g. the necessary logic to disable the motherboard CPU. The A500 (and therefore the Checkmate board) doesn't have this logic, so you'd probably have to remove the onboard CPU for it to work, or else add that logic on an adaptor board as A2000 accelerators would probably make use of this functionality.
But the biggest thing to consider is that the pins on the Checkmate A500 passthrough slot are mirrored. You'd need any adaptor board to take that into account, otherwise plugging an A2000 accelerator into the slot will connect pin 1-2, pin 2-1, pin 9-10, pin 10-9 and so on, which will likely let the magic smoke out of whatever gets 12V in through a clock line. As terriblefire said, this wasn't considered an issue at the time because A2000 accelerators were much too big to fit in the Checkmate case, but hey ho