Seedless grapes are available here. It`s just a matter of seasons and knowing which ones are what. Fruits and vegetables in Japan actually tend to follow a natural seasonal pattern when it comes to availability.
Later this month you should start spotting seedless varieties. They grow a lot of them in Nagano.
BUT, I really cannot recommend them for a quick snack while you`re out. In fact, I can`t recommend any fruit for a quick snack while you`re out, unless you have money to burn. A single bunch of grapes can hit 1000 with no problem. The price on most fruit is painful.
When it comes to losing weight on a Japanese diet - I think it really depends on the person. Rice converts into sugar, and if your body isn`t fast at metabolizing it you`ll gain tons of weight. I seriously can`t eat more than a spoon or two full of rice a day without gaining.