Ok I checked at work and I still have the SD card on which I had Slackware running on the Pi.
Unfortunately at the time I tried none of the community images booted on the device I had lent to me so I did it according to what I documented as emergency way of getting slackware on the Pi here
below the community links.
Anyway on the boot partition in my SD did not have an initrd, not sure if fatdog's image has:
if it does you will not need to edit config.txt,
if it does not you should add something like this to config.txt "initramfs initrd.gz 0x00800000"
You will then haveto create an initrd image. If you already have initrd you will need to figure out what options it was built with and add the lvm modules via -m and enable lvm root via -L, if fatdog's image does not use initrd you will only need to create initrd with "-m dm-mod -L" options but do checkout if dm-mod has dependencies as on my kernel build it's built into the kernel.
Hope this helps more the the previous my post.