Here is an Upper Body Torso Belt
http://www.gforce.com/products/netsanres/4290.php
In years past, They were sold by the popular Autocrosser named Dick Turner. For that reason they picked up the name of, "the Dick Strap."
If you have ever used one they can be a good, cheap ($17) alternative to a harness for autocrossing. The proper use is to place the entire strap around the drivers chest and behind the seat. The clasp should be right at your chest, going under your arms and keeping your shoulder blades planted against the seat. Unfortunately, if you use one in an open top car (this includes both convertibles or cars with the t-tops or sunroof removed) then you are required to run a roll bar that clears the helmet. Conversely, the strap can be used in any hard top car (or car with the t-tops or sunroof left in place while competing) with no modifications.
Yes, a lap belt is not "upper body restraint" but a Dick Strap is.
What the rule does not clearly state is if you can run a convertable with the top up along with upper body? The rule does state that you can run a t-top car with the t-tops in place to avoid the requirement.
Repeat of rule:
3.3.1 Driver Restraints
Seat lap belts are required in all cars, and must be installed in cars
with passive restraint systems that do not include a lap belt. Installation
and the use of shoulder belts or harnesses is strongly recommended,
however non-factory upper body restraints may only be
used in open cars, cars with targa-tops in the open position, or cars
with T-tops in the open position when two conditions are met:
a. The roll structure must meet either the requirements of Appendix
C or Section 9.4 of the GCR.
b. The top of the roll structure may not be below the top of the
driver’s helmet when the driver is in the normal driving position.
Personally, in my Miata, I can not fit with a helmet and roof closed and even if I put a roll bar I can not keep the helmet under the bar. Because of this, I can not run upper body restraint. I have not tried altering the seat by taking out foam.