In Charles’ latest video update (released last week) he does mention this paper and says that its soon to be released.
IOHK has recently an internal paper that will be released soon on a Byzantine Fault Tolerant protocol that was inspired by and complementary to Ouroboros.
Taken from the summary of his video update, which you can find here: 07.09.2018 - Charles Hoskinson Video Update - #4 by maki.mukai
And as soon as its published, I’m sure IOHK will share it across their social channels or you can check back onto the IOHK Papers page which was linked above by canopus. 