Create a RAID-1 array, md0. Then do the following:

mke2fs /dev/md0
e2label /dev/md0 foo
add to /etc/fstab:
 LABEL=foo /misc ext3 defaults 1 1
mount /misc
<look at filesystem>

umount /misc
pvcreate /dev/md0
vgcreate wibble /dev/md0
lvcreate --size=<whatever> -name wow bar
mke2fs /dev/bar/wow
e2label /dev/bar/wow foo

Then:

# mount /misc
mount: error: the label foo occurs on both md0 and dm-0

Shouldn't making the LVM volume get rid of the old filesystem label
somehow?  You can always run 'e2label /dev/md0 ""' to delete the
label, but running that on the the raw RAID volume under the LVM seems
unwise.