I read somewhere that kids pick their noses because something is irritating them up there, and they don't have the impulse control to get a tissue instead of a finger, which is much more handy (and gross!). It's not that they like the feel of picking so much as they dislike the feeling if they don't. That kind of changed my approach to nose-picking.
I have focused on keeping kleenex boxes all over the place, and even giving a child a few tissues for his pocket. It does seem to help.
If your child is hiding to do it, that may not help. But maybe approaching it less judgementally can help him to come out of hiding. So not the "Ew, gross! Stop that!" that we're all prone to shout out when we see nose-picking. Instead saying calmly, "It looks like something is bothering you. How about a tissue, sweetie?"
Maybe?
