Just discussed this on another thread. Yes, they routinely sell out business class to Hawaii. However, the fares are much lower than business class fares to the mainland US. A lot of the demand comes through tour operators who buy the seats in bulk and sell them to honeymooners etc. There is a huge gap even in the published fares. From a quick Google search, JL business class r/t from HND to HNL later this month (7.5 hour flight eastbound) prices at $2,439. To SFO on the same dates (9.5 hour flight eastbound), the fare is $4,835.