As long as the bar is made from the correct grade of steel and not silly thin tube I fail to see why there would ever be a problem. Better that you are strapped to thick tube than thin sheet steel like the parcel shelf.
That's a bit logical for the MSA though Dave...!!
I can't be bothered to give myself a headache reading the Blur Book today, but I think you're right & as long as the correct grade steel tube is used it's fine - I'm sure there's a reg. in there somewhere...!! But I also seem to recall it's only a harness bar that can be added to a homologated cage unless any other additions are also homologated by the cage manufacturer?