Do belly putter grips fit on standard length putter shafts?

yes they do. the shaft is the same diameter. tou have come up with a very good original thought. using the longer grip will help using putter from off the green. also for sand lob wedge. longer grip will make half shots much easier. on the wedges use round grips. it does work. you will find that you can use full putter grip length for longer putts. choke down a touch short putting will improve. very well thought out. well done. your scores will drop. full marks .