Most instructions I've found involving force carbonating a home brew assume the use of a Corney keg. In these instructions, I always see it suggested that one should attach the gas line to the beverage port so that the gas is introduced directly into the liquid.
I haven't been able to find consistent instructions for force carbonating a Sanke or Sixth Barrell keg. It seems like there are a number of people who suggest a similar procedure as the Corney keg where the gas is attached to the top portion of the Sanke coupler to force the gas down the spear, but not everyone seems to do so. There are others that suggest hooking up the gas like normal. If one were to reverse the lines, would they need to remove the check ball from the coupler? It's also unclear to me how one would purge the keg to remove oxygen. The only info I could find was to fill the tank with CO2 before racking.