Just recompiled everything with yesterday's source just in case. I'm
pretty sure this used to work:

quartz# cd releasedir/i386/installation/floppy
quartz# ls
BSDSUM          SYSVSUM         boot-ps2-1.fs   boot-tiny.fs    bootlap1.fs
CKSUM           boot-big.fs     boot-ps2-2.fs   boot1.fs        bootlap2.fs
MD5             boot-com1.fs    boot-small1.fs  boot2.fs        rescue-tiny.fs
SHA512          boot-com2.fs    boot-small2.fs  bootlap-big.fs
quartz# vnconfig -l
vnd0: not in use
vnd1: not in use
vnd2: not in use
vnd3: not in use
quartz# vnconfig vnd0 boot-big.fs
quartz# disklabel vnd0
# /dev/rvnd0d:
type: vnd
disk: vnd
label: fictitious
flags:
bytes/sector: 512
sectors/track: 32
tracks/cylinder: 64
sectors/cylinder: 2048
cylinders: 2
total sectors: 5760
rpm: 3600
interleave: 1
trackskew: 0
cylinderskew: 0
headswitch: 0           # microseconds
track-to-track seek: 0  # microseconds
drivedata: 0 

4 partitions:
#        size    offset     fstype [fsize bsize cpg/sgs]
 a:      5760         0     4.2BSD      0     0     0  # (Cyl.      0 -      2*)
 d:      5760         0     unused      0     0        # (Cyl.      0 -      2*)
disklabel: boot block size 0
disklabel: super block size 0
quartz# mount /dev/vnd0a /mnt
mount_ffs: /dev/vnd0a on /mnt: incorrect super block
quartz# mount /dev/vnd0d /mnt
mount_ffs: /dev/vnd0d on /mnt: incorrect super block
quartz# mount -t ffs /dev/vnd0d /mnt
mount_ffs: /dev/vnd0d on /mnt: incorrect super block
quartz# hexdump -C boot-big.fs | head -n 5
00000000  eb 3c 90 4e 65 74 42 53  44 32 30 00 00 00 00 00  |ë<.NetBSD20.....|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 31 c0  |..............1À|
00000040  8e d8 8e c0 8e d0 bc fc  ff 52 cd 13 5a b9 01 00  |.Ø.À.мüÿRÍ.Z¹..|
quartz#


Shouldn't that work?

Cheers,

Patrick