Making any changes to the disks with Casper Imaging launched will most likely cause issues when trying to image subsequently. Imaging reads what drives are currently mounted during startup, and splitting the drive will likely change the location of the mounted disks. The safest route would be to run a script to split the drive before launching Imaging or relaunch Imaging after the split has been performed.
#!/bin/bash
#Breaks the coreStorage volume and names the non-ssd drive "Macintosh HD"
CoreStorage=$(diskutil coreStorage list)
if [ "$CoreStorage" != "No CoreStorage logical volume groups found" ]; then
diskutil coreStorage delete $(diskutil coreStorage list | grep "Logical Volume Group" | awk '{print $5}')
diskutil eraseDisk JHFS+ temp /dev/disk0
diskutil eraseDisk JHFS+ Macintosh HD /dev/disk1
sleep 10
fi
save as script, run as a startup script in your autocasper-netboot image.
this assumes only one disk in the machine.