Doug give Klipan/Autoliv in Melbourne a call, they make FIA certified 4,5 & 6 point harnesses. They also de tests of seats and other components in relation to FIA specs if you need certification. They should be able to give you a recommended practice for mounting the belt.
my preference would be to wrap it directly around the tube as its lighter and keeps the forces more in line, however this is just a personal view.
Could you make the bar diameter smaller, but thicker walled, that way it would be spaced of the firewall enough to get the belt through?
Noticed it on the locost forum but not oz cubbies – maybe some one there would have better info regards rego and local motor sport requirements.
Regards Ryan