Sure is. Brey Krause Harness Bar IS legal for Super Stock for the purpose of mounting the harness to it.
Solo II GCR 13.2.H .... A horizontal "harness bar" may be used as part of the installation hardware for allowed driver restraints. .....
BUT, you most mount a harness to it and wear the harness for it to be legal
BTW, a 4 point welded roll bar is also legal. a 6 point welded in roll cage is not but a bolted in one is.