In part that will depend upon its ETOPS capabilities, i.e. how far it can be trusted to fly after an engine fails. The Great Circle mapper is fun for playing "what if" games with potential routes: e.g. here's the direct Seattle-Tokyo routing with dark shading showing parts where flying for 60 mins at 410 knots wouldn't reach an airport. So if that was the ETOPS performance for a Boom aircraft (and I've no idea; I just picked a B777 as an example) then the route would have pass a little closer to Alaska and Sakhalin to keep the possibility of a safe diversion at all times, and that in turn might make the route too long.

http://www.gcmap.com/mapui?P=KSEA-RJAA&MS=wls&DU=mi&E=60&EV=...