Testing with the 2013-01-10 15:41 winuae.exe and A4000 emulation the installer boots further. Fingers crossed, it might even actually install fully; but I haven't tried that yet.
Update: Installation might be possible via FTP and WinUAE's A2065 emulation, but I was running under Wine where that probably won't work. So I created an Amiga FFS partition on the HD and copied the NetBSD installation files there. On booting next time, after specifying the Amiga FFS partition the NetBSD installer reported:
mount_ados: /dev/wd0d on /mnt2: Device busy
Cannot mount disk. Aborting.
The FFS partition I created uses 1024-byte blocks. I'll try reformatting it to use 512-byte blocks in case that was the cause of the problem. Edit: it wasn't, same happens with 512-byte-block FFS partition.
If anyone wants to test this without the hassle of copying the NetBSD files to the HD first, I will upload an archive containing my HD image. It's about 162MB in size. Use the NetBSD_install_A4000.uae config file above with it.
http://www..com/archive/LYteB...-01-10tar.html