After much head-scratching and code optimisation, I finally have collision detection working! In this version you can shoot the ring segments. For now they only take one hit, but this can easily be changed.
I think I now have all of the pieces necessary to make the game. However, I now need to put it all together and rewrite significant parts of it, such as the kernel. The main issue now is running out of ROM space as all the unrolled code has nearly used up the full 12KB.
Let me know if you have any suggestions, or if you find any collision detection bugs.
P.S. Remember to hit Alt-P in Stella to avoid the flicker.