I have no solution, I can only relate, My son picked his nose daily, it would bleed but he'd still pick, I swear it was a comfort like a child sucking his thumb, he was just comfortable with his finger up his nose, He picked for about a full year. But then, one day, it ended.
I never criticized him for it. I just told him "Just so you know, if you decide to pick your nose in public, some people may find it offensive so dont be surprised if someone says something". So I saw him being cautious in public but I'd still find it up there.
I would always hand him a tissue every time I'd see his finger up his nose. I just really felt it was a phase he was going through, and even though I'd never heard of other peoples kids doing it, I didnt feel my son was the only one, like there was something wrong, I felt confident he'd grow out of it. And I'm glad I never said anything (that's just me personally) because now, he doesnt even remember that he did it, so he doesnt associate him self with that, I didnt make a big deal out of it, I was more concerned about his self esteme in the long run than his nose at the time.
But then he went on to some other quirkey habit (lol), but atleast it wasnt nose picking. He's just been such a joy to watch grow and develope.