In the case of Boston and a couple other JAL 787 routes, namely San Francisco, Frankfurt and Moscow, a four seat F cabin would probably work, along with a modest decrease in the number of J seats and some premium economy. Not that I have any expectation of the configuration changing, but creating a subfleet for a few specific routes like that can get hugely problematic when weather and mechanical issues interfere. When BA rolled out their new F product they naturally attempted to concentrate it on their most lucrative routes - New York, Hong Kong and Sao Paulo. When all was going smoothly it was fine but once things got out of whack they were subbing in old F left and right and it took them days to catch up. The old F product, which is still in use on about a third of their 747s, is a perfectly good product but it's not up to the same standard and it's not what those customers were expecting. Having seen plenty of equipment substitutions where people have been downgraded to a lower class or didn't get the product they expected, I can say it's not generally a good thing for an airline to get in the habit of doing. Usually seven out of ten people take the compensation and deal with it but there are always a few who will make a stink. Unfortunately, when it's a guy who flies paid F back and forth to London fifteen or twenty times a year, you kind of have to go out of your way to ensure that he remains a happy, loyal customer.
