An interesting paper has been made available as pre-print: "Peerlock:
Flexsealing BGP" by Tyler McDaniel, Jared M. Smith, and Max Schuchard
from the University of Tennessee.
The paper probably is the most formal description of Peerlock so far.
They even conducted active measurements to reverse-engineer what the
state of Peerlock deployment is in the global Intenet routing system.