LOL - we are working on nail biting and haven't even gotten to DS's horrible nose picking habit yet. Have you tried putting band aids on his fingers so that he can catch himself picking his nose? Maybe he does not realize it. I imagine you could use some of the same techniques as nail biting - give him something else to do with his hands besides pick his nose.
I think your strategy is right on - no yelling, discipline, shame etc. over this. It's a bad habit and hopefully he will grow out of it sooner than later.