[accurate explanation of disklabel problems snipped]

>Is it impossible for you to go the way for 'network boot + network install'?

The other approach is to use the NetBSD/i386 machine to dd the 32M
diskimage onto a disk. (Use the entire-disk partition: sd?d on
netBSD/i386.)

That creates a bootable NetbSD/pmax disk.  Then just move the disk to
the 3100 and install the tarballs via NFs or FTP.