Well, you have a very tight space constraint. It can be asked if AROS KS ROM even fits 512KB as it is now. Images are definitely out, so anything you do must be mathematically generated.
I don't think that AROS even implements the "please insert disk" animation of any sort as of now. So you have free hands on this one. As long as your effects fits into say 7KB it should be fine. Some free hints: You really want to look into vector graphics and perhaps procedural generation.
I would like something like Loonies' Luminagia or Ikadalawampu as boot screen ;P
