Actually, I'm pretty sure that debuting on a CD does nothing to the chance of getting in the game. Kosmos Cosmos, Meisou Mind, Me ga Au Toki, I Want, Futari no Kioku, Tonari Ni..., Kiramekirari, Ippai Ippai, Starto Star, Furu Furu Future, and i all started in the Master Artist CD's. Kamisama no Birthday started on a Masterwork 00, Overmaster and Colorful Days both started with singles albums released just before SP came out, and LOBM originated in the Master Special series. That's a pretty good chunk of CD-first songs.
Now, I WILL say that recent CD-debut songs haven't made it on, such as Next Life, Flower Girl, and Arcadia, but that's not to say that they'll NEVER get on. The next DLC is planned to release a whopping TWO songs, so who knows, maybe they'll use that model to release the Master Special image songs?