kroack, your curiosty wants you to get your hands on a North Atlantic MNPSA Operations Manual.
You'll find the procedures for comms failure in Chapter 7 Communications and Positions Reporting Procedures. It's a bit lengthy and verbose so I won't reproduce it entirely.Mine says:
"it is strongly recommended that a pilot experiencing communications failure whils still in domestic airspace does not enter OCA but adopts the procedure specified in the appropriate domestic AIP and lands at a suitable airport. However, if the pilot elects to continue, then, to allow ATC to provide adequate separation:
(1) if operating with a received and acknowledged Oceanic Clearance [... fly that clearance..]
(2) if operating without a received and acknowledged Oceanic Clearance, [... fly the entrypoint, level and speed as filed in the file plan...]. The initial oceanic level and speed must be maintaned until landfall."