Nevermind; I figured out what I did wrong.  Apparently the vnd driver
doesn't work with files that have 'holes' in them.  The command:

# dd if=/dev/zero of=test-2kbps.vdsk bs=2048 count=1 seek=511

should have been:

# dd if=/dev/zero of=test-2kbps.vdsk bs=2048 count=512

After doing this, I was able to create a disklabel when the file was
configured as a 512-bytes-per-sector virtual disk.  When configured at 2048
bytes per sector, disklabel gave no errors, but no label was ever written
to the disk.  Now I have a good starting point and test environment for
figuring out why.

P.S.: Does anybody know if it's at all possible to run a kernel debugger
remotely over a network connection?

-----------------------
Trevin Beattie          "Do not meddle in the affairs of wizards,
trevin@xmission.com     for you are crunchy and good with ketchup."
      {:->                                     --unknown