EDIT: Never mind.
reading the revised lores makes it clear, IMO, that the comet can not be dispelled by any magical item/spell that generically removes RIP spells. It has to be specifically dispelled during either the casting( on your turn) or in the opponents magic phase.

Hiya. I was just thinking about the magic item Lizardmen got, Cube of darkness. I guess I can't quote the entire text about it for copyright reasons but in short it does the following: end all RIP and 50% chance of ending the current magic phase.
Now, that sounds real good when you've gotten off a comet of Casandora right? Use the cube to prevent the opponent from dispelling the comet. He/she has to wait two magic phases until the chance to dispel it comes again. At which point the comet has at either 2 more counters or has already dropped from the sky and pummeled the targeted troops.

Now, what with the comet and the cube? Does the cube remove the comet along with every other RIP? Does the comet count as a 'Remain In Play'?

Kinda ruins the moment if it does....anyone out there care to take a guess at the legality of the above combo?