Interestingly enough, right after installing NetBSD and booting DOS, fdisk
   and pfdisk both tell me that there's a single 24M partition (50,000 sectors)
   at the beginning of the disk with the 165 sys_id.

When you tell disklabel(8) to overwrite the DOS partition table, it
installs a fake one, with a bogus geometry.  It doesn't matter whether
or not it's correct, because all it needs to do is load the NetBSD
boot program.

However, it won't trash the partition table unless you tell it to.