I've seen it done a few times, but I'm not sure about the seat rails. I don't think that there is anything commercially available. You'd have to fab your own. For a harness the best place to mount it is to a roll bar/cage or at the least a harness bar. Again, your looking at custom stuff or the Autopower bolt in.
from now on i just go find u and just let u install everything... its soo hard to find stuffs for this car... seats are easy to find ... but puttin them on and the harness its just hard!!!
Just a update on this thread..
Here is the actual seat that I made custom mounting rail for a sparco.
Using a old MKI seat rails so that seat can still be adjusted front and back
Licensed Auto Technician, Gov Inspector, AirCare, Pressure Fuels Installer Mr2 Parts