I first worked on this driver last year, after Pierpaolo started it. (It's currently playable, but there are plenty of graphics glitches.) Unfortunately, I was unable to make any further progress without access to the physical hardware.
So, now I have the Space Raider board on my workbench, and I'm slowly making progress. It's amazing what one person can do with a multimeter, some databooks, and time. :)
Right now I'm concentrating on the I/O section for the sound cpu. In case anyone's interested, this is how I'm proceeding. First, I found the 5 sound chips and backtraced them to the address decoder for all the IO ports. Now I'm going through ports 0x28-0x2f, 0x30, and 0x38 and figuring out which bits are being used and for what purpose. Ports 0x30 and 0x38 go to a bank of 8 4051 analog mux IC's. They each have graduated resistor ladders connected to them. Maybe they are a fancy volume control or an analog filter. I should be able to get that figured out soon.
Additionally, I'm making up a harness to use to power up the board. I discovered jammaboards.com, and they had everything I needed for <$15. I'm hopeful that I can get a lot more information by actually playing the game! :) It will be very nice to get the grid behavior correct.