Finally threw together my plipbox and put the 0,6 firmware into it.
Tested it with the 68000 version of both Miami and Roadshow, and it works great!
AmiFTP transfers from an FTP server on the local network is about 64-70KB per second, with 68HC000 clocked at 28MHz. That transfer speed is about 20 times faster than my serial PPP solution, so I'm pretty much happy with it!
But I notice that if I reboot the Amiga the plip fails to get an internet connection after reboot and stack startup unless itself is also reset (I push the reset button on the Arduino).