Right of a single bar falls under the discourage category. Right of a double bar is prohibited.
B31.3-2006 is all I have handy, and it's just a single bar at 800 with no double bar to be found.
So, I still stand by my "does not prohibit" statement.
If sigma needs anything more comprehensive than a hint to go look at B31.3 or Sec2... well, a free message board probably isn't the place to get it. Even us burned out rock stars turned engineers need some business nowadays.
-SP