Radical Dreamers originally contained one easter egg; Demiforce added two more.
Unidentified Flying Objects
If you play the scenario SuperXtreme Alphacosmos Police Case EX Ultra, the ending screen will feature UFOs in place of the pink birds that normally fly.
To access the music test, first beat Le Trésor Interdit. On a new round, fall asleep when given the choice, then go to the study and examine the desk. Head to Riddel's room and go through her drawer until the text "if only I had more options" appears, followed by two options. Hit down twice and the cursor will disappear to a secret, third option. Press confirm, and the music test will appear. After you are done listening to the tracks, exit out and finish the scene (Magil: Caught Between Love and Adventure). On the score screen, "Bonus: Music Test" will be checked.
All Your Base
Repeat the steps to get to the music test, and highlight song 6. Select it, and on the next screen, press down twice to reach a secret option. Confirm it, then select song 6 again, and press down twice to reach the secret option once more. Confirm it, and repeat the process until a cutscene ensues parodying the AYB meme.