Thought I share my tutorial (youtube) on how to install Slackware on an encrypted partition using LVM on LUKS. Opinions are welcome!
But to summarize it the trickiest part is the initrd which you'll make using the following command:
# mkinitd -c -k 4.4.14 -m ext4 -f ext4 -r /dev/VGNAME/LVNAME -C /dev/sdXY -L
Replace "ext4" with your filesystem of choice for the root partition.
And make changes to LiLo so that it can recognize the encrypted partition and unlock it.