Neat! I don't have an A600, but I think this is a very cool idea. I hope to see it spread to other classic accelerators. What speeds do you think you can achieve with which FPGA?
At the risk of feature creep, perhaps once things are working well you can add an SODIMM slot as well and some other ports to be made use of later if there's room left in FPGA for their controllers? (IDE, USB, whatever)