Absolutely. Remember the number of times Billy Graham preached behind the Iron Curtain? We'll never know what role that had in eroding and undercutting communism's strength there, but a book called "Candles Behind the Wall" suggests that it was considerable.
I'm not trying to be disrespectful of Rick Warren. In today's world, anyone working to bring people closer to God and further from selfishness, greed, worldliness and carnality is doing God's work.
The difference in terms of the Billy Graham example you cited when he went to the USSR is that there was already a Christian tradition insider of Russia, including a fairly sizeable Protestant presence outside of the official Orthodox faith. There is nothing like that in North Korea which never had a Christian presence and all ideas of there being a higher power beyond the state have long since been eliminated. Russia only half-heartedly tried that, and mostly under Stalin. But NK has thoroughly brainwashed their people with a cult of personality around the Kim family. I'm just not sure there's any penetrating that, nor any willingness to allow it to so be by the government. Anyway, I wish him luck. I'm just not holding my breath.