The script seems to do precisely the same thing on OpenBSD but with 
positive results:

It runs:

# fdisk -c $cylinders -h $tracks_cylinder -s $sectors_track \
     -f $root_dir/$MBR_TEMPLATE -e $vnode_device << __EOF
reinit
update
write
quit
__EOF


Which results in:

fdisk: sysctl(machdep.bios.diskinfo): Device not configured
Disk: svnd0     geometry: 1999/16/63 [2014992 Sectors]
Offset: 0       Signature: 0xAA55
          Starting       Ending       LBA Info:
  #: id    C   H  S -    C   H  S [       start:      size   ]
------------------------------------------------------------------------
  0: 00    0   0  0 -    0   0  0 [           0:           0 ] unused
  1: 00    0   0  0 -    0   0  0 [           0:           0 ] unused
  2: 00    0   0  0 -    0   0  0 [           0:           0 ] unused
*3: A6    0   1  1 - 1998  15 63 [          63:     2014929 ] OpenBSD


The Start and Size are the same as mine (I think).  The only difference is 
that they're still using Partition 3 by default and we're using 0 now 
(when did that start anyway?)

I'll look in the morning.  Maybe I'm doing something wrong with 
installboot(8), but I don't think so.

~BAS