I've got a _big_ problem.  I finally got an old Archive
  tape working on my i386 system at home, running NetBSD-i386
  1.1.

You have my sympathy.

You would not normally use the block device on a tape drive.  The raw device
is much faster and doesn't thrash your disk block cache.  I suspect that is
why no one has caught this conflict before you.

If you really overwrote the first 30 MB of your file system, I think it's
hopeless.  That's where all the important stuff is.

For jumpers, you might check the UM Apollo Archive.  Old Apollo computers
used an Archive qic controller, but I don't know the model number.  The UM
Apollo Archive is at http://pisa.citi.umich.edu/.

        I don't know enough about device drivers to
        be _sure_ that conflicts are a bad thing, but if
        conflicts were okay, we could just make everything be
        major 0, minor 0!

I once made such a system by accident, using the wrong version of tar to
restore a root file system.  Interestingly enough it actually ran, but
exhibited all kinds of strange behavior.