I connect my gas to the beer out post, set my reg to about the same pressures you're talking about (140kPa / 6psi) and rock the keg about vigorously. Just like shaking a bottle of coke lets the pressure out in a low pressure environment, this lets the gas in in a high pressure environment. You will hear the gas 'singing' as it goes in. After about 10 or so minutes you're done - don't forget swap your disconnects back.
This is a useful technique to have in your arsenal in case you need to carb up a keg at short notice for a party. As I usually end up carbing this way anyway, I have threaded disconnects rather than barbs to make swapping them over easy to do for gassing though the liquid post.