Okay, so the last main part of my burst of sprite-enhancements is now working: bitplane mode for hires sprites. What can be seen below is two sprites on screen, fully coloured in (all one bits for the image), overlapping each other and a character-mode checkerboard in the background. The presence of each sprite causes one bit in the output colour to be flipped, and as this occurs in each combination, it yields a distinct colour.
For a lame piece of basic, it looks kind of interesting. Of course it would look better with some interesting animation. I might have some fun later finding some games where turning on bitplane mode on the sprites yields pretty results, but that will have to wait until I have more time. For now I am happy that I have implemented the main outstanding sprite functions -- although the collision detection still has some wrinkles to be worked out.