Well, if you look at line 35 of Scene_Title, you will see:
@sprite.bitmap = RPG::Cache.title($data_system.title_name)
Something like the following would do the trick:
random_titles = ["Title1", "Title2", ..., "TitleN"]
@sprite.bitmap = RPG::Cache.title(random_titles[rand (random_titles.size)])
Where Title1, ... TitleN are the names of actual graphics located in the Titles folder of Graphics.