I think this is a wonderful idea. Not just a classy and visually elegant body accessory, a bellybutton piercing is also a statement of individuality and an assertion of independence. Provided that you train exclusively in 10th Planet jiujitsu (and none of those other dojos, booo) you'll be fine, as you should be aware that gripping piercings is completely legal in gi jiujitsu and is especially encouraged by the IBJJF. Sometimes it can help you while defending a kimura to grab onto it yourself, or to secure spider web, but in general it will be to your detriment. Plus, people with a less nuanced understanding of grappling rulesets will accuse you of cheating; one of the main reasons I stopped training in the gi is because people always said it was "cheap" and a "strong guy's defense" to hold on to my prince albert. In a way, I suppose they're right.