Well, I found time to make a few more discoveries. The Space Raider boardset is 3 PCB's. Two of them are full size and obviously two halves of the same design. Then, there is a little PCB grafted on top, connected via a 24-pin dip connector, with wires going to the other two boards.
I suspected that this board was the starfield generator. Sure enough, if you unplug it and run the board, the stars disappear. Cool - less work for me.
Then, I noticed something else. Two chips were missing from the board, and it was labeled with the number 8011, unlike the other boards. Maybe this board is from another game? Searching through the MAME code, there is a comment that Universal game #8011 is Zero Hour.
Luckily, a manual and schematics are available for Zero Hour. The starfield board is the same! In Zero Hour, this board provides both color generation and stars. The missing chips in Space Raider disable the color generation part, since that's already on the main board.
Anyways, I added preliminary support for the scrolling stars to MAME, and it looks right. I need to measure a few more things to get the timing exact. When I'm done with Space Raider, I'd like to add the stars to the Zero Hour / Red Clash driver, since it's missing there as well.
As a side note - I hope these posts are interesting to someone. I think it's kinda cool to document the little things that it takes to get a MAME driver right.