I understand what you are saying about your LVM investment,
but placing the /root lv from two different systems on the same VG is not a good idea.
LVM is at a different version as well as the tools.
boot up in rescue mode
vgchange -a y <implied all vg's)
this will make all your vg's available
then do a vgs
If you see vg(s)
then chroot to the img (of fedora
play around and check things out like
mount -a #<= mount everything in fstab
is there a diference, that would be good.
edit /boot/grub/menu.list to be like this (using your boot partition, root vg-lg)
title Fedora Core
root (hd0,0) #<= whatever /boot is on
kernel /vmlinuz-2.6.18-1.2798.fc6 ro root=/dev/VolGroup00/LogVol00 (or 01?) rhgb quiet
That's all I know.