That would be HOT. I'd love to plug such a thing into my A500. I always wondered why someone didn't just make a board which went into the 1200's accelerator slot, had a USB2 cable at the other end, and essentially made the 1200 a slave to the modern machine on the other end. USB2's bandwidth exceeds anything happening inside the 1200, it could just see the other box as a really fast accelerator.
USB2 has a theoretical peak bandwidth of up to 480Mb/s (megabits/sec). In actual use you'll not see that sort of speed. My USB2.0 hard disk can pump data at around 30MB/s, which is certainly not faster than (for example) local memory speed on a good accelerator card.
Of course, the main problem with USB is not so much the theoretical bandwidth but the latency. USB is a serial, packet based protocol, pretty much designed for streaming data between devices. Latency there is not that big an issue, a few milliseconds point to point lag doesn't matter as long as the packets keep transferring at a continuous rate. That's not true for your A1200 trapdoor where you have dozens of independent (and interdependent) signals to worry about. Hardware interrupts and the like don't need to be held up by being encapsulated into a USB packet, sent down a wire to a host, which in turn will be interrupted to deal with it and so on (yes I realise USB has interrupt and control transfers but I don't think their timing requirements are necessarily suitable for dealing with stuff happening inside your miggy).
