You can use exactly the same method.
dd if=/dev/VolGroup00/LogVol00 of=...etc
dd if=/dev/mapper/VolGroup00-LogVol00 of=...etc
However, if this is a running system, you might like to investigate one of the fantastic features of LVM == snapshotting!
You need a bit of empty space in the volume group. (If there's none free at the moment you need to resize the logical volume AND the filesystem using a rescue/live cd).
The you just do something like:
#lvcreate -s -L 64M -n LogVol00-snapshot /dev/VolGroup00/LogVol00
(as the snapshot is 'copy-on-write' the -L size needs only be as big as the changes that occur while using the snapshot ... plus a bit
This creates a snapshot at /dev/VolGroup00/LogVol00-snapshot that looks the same size as the original.
Then you can use your dd method for imaging.
To remove after: