There are also no regulations about cruise clearances. A cruise clearance is different from a block altitude clearance in that it includes an approach clearance and once you report descending out of an altitude you can't go back up to that altitude. Those differences are pointed out in the AIM which says in its preface that it is not regulatory. So if a pilot failed to comply with the conditions of a cruise clearance they would be violating 91.123 and not some cruise clearance regulation. In 40+ years of IFR flying I've only received a cruise clearance one time between Maui and Kona in the middle of the night.