This isn't a US vs Japan thing....Takata themselves say not to mount a harness that way. Looks like it doesn't fit the JAF rules from the pdf you posted either.
This is exactly what pisses me off about that car (and other's like it). Yes it's got some really interesting engineering and looks really cool. But it is clearly unsafe (regardless of rules) and doesn't follow any recommended safety guidelines anyway (US or Japan). But...it's TRD/Toyota so most just assume that it's okay. People will defend it because they think such a big name couldn't possibly do something wrong. And they'll copy it because they saw it on Speedhunters and put themselves in danger. So much of what you see at the track is people copying other people that look like they know what they're doing. That's usually fine, but in this case it's very bad.
Anyway, build a light car but budget a few pounds in for reasonable safety equipment. Sorry for threadjack and rant.
- Andrew