SO, FOR LACK of anything better to work on, I decided to see if I could actually get a kernel working for a VCS Metroid.
The answer is yes.
Right now it supports a partially asymmetric, striped playfield (PF1 & PF2), one non-flickered sprite, up to five (limited by RAM) intelliflickered sprites, M0 and M1.
I'm kind of amazed that I got it working, actually. Some of the missile writes come at non-optimal times (around repositioning scanlines) but not too bad.