@fatbunny I think I would take this approach. Recreate your master image using the smallest disk possible. Make sure the root lvm volume is the last one allocated on the disk. Capture with FOG it will still capture as RAW. Then using a post install script that detects either the image name or linux issue the LVM commands to extend the lvm volume to the size of the (new) disk and then extend the root lvm volume to the size of the lvm disk. Its a bit strange on how to handle it, but it should work.
The basic idea is to create your source image as small as possible then expand it post deployment.