I doubt it's a problem. The body is nourished through the mouth, and when bad or tainted things constantly are in it, the whole body suffers, so a new and worthless object introduced to the tongue would be a concern.
On the other hand, all of us are generally through with our navels after the umbilical cord is cut -- it's usually a matter of whether it's clean for those we welcome near it.
Just to be sure...If you look like Dawn Wells, Raquel Welch, or Barbara Eden, I would be more than willing to check your navel out. ; )