Kernel revisions panned out - the ball object can now be resized and recolored.
The draw missile routines haven't been updated yet to use the ball as a last resort, so to test the resizing I've temporarily modified the stars to have different widths. I also added some missiles that fly around in a formation just below the enemy formation (they're easy to spot in motion).
I see some stray graphics every once in a while that I'll need to track down, such as the 2 purplish dots at the bottom-left of the station:
I've narrowed it down to sprites interfering with other sprites as the problem still occurs if I disable the shot and starfield routines:
I suspect a player is getting reused before it should be as some shifting of the datastream usage has occurred.
ROM requires a Harmony Cart or Stella 5.0.0-pre6 or newer.