FYI, I am working on it. To get it working we'll first have it boot properly from 64 mode and in order to do that the 158 driver needs to be ported to Taking advance of the SD controller. I've managed to strip away the turbo code so at this point no true 1581 drive emulation isn't needed any longer. I will now commence to replace block transfer code with logic that talks to the SD controller directly. For that I need to setup the Nexys board as a proper devkit with etherload and serial debug access as until this point I've been working through VICE.
158 driver -> 1581 driver