Scratched after 8 days.
I spoke to Speedware Motorsports about their harness bar -- they stated it is fairly easy to install. Since I'll be in their vicinity late July, if everything goes as planned I'll get it installed in my new STi, perhaps with a new seat. According to them, once it is installed, you can take out the center bar quite easily, and the support struts then fold down next to the seats, making it very practical.
Every time I go on an autocross weekend (Friday test'n'tune being 20-50 runs, Sat & Sun 6-8 runs each day) I end up with bruises on both legs from bracing myself. Since my car isn't a G monster, I suspect it would be far worse with the STi (which is probably a lot more spacious inside than the Miata anyway, making bracing even more problematic).