The UA configuration you may be referring to are the handful of two-class domestic 777's UA uses primarily for mainland-Hawaii flying. They don't use these in trans-PAC operations. So it seems the viability is pretty good.
A little understanding of UA's situation is in order.
The only reason they don't use a two class 777 to NRT is because ALPO won't allow it. They find the crew rest accommodations on the two class 777 to be unacceptable, but gave a route specific excemption for ORD-OGG/HNL because of the embarassment they were causing UA (i.e. AA was able to fly to Hawaii from UA's hometown, yet UA couldn't).
UA has the same problem with the 763, so unless they want to rip out seats and put in a bunkhouse, UA has no choice on any route over 8 hours but to operate with the three class 777 or 744.
AirLuver, BOS-NRT and JFK-NRT were awarded in the same route award (OST-1998-3419) that allowed AA to serve ORD-NRT. BOS was deferred awaiting slots, and ultimately, the some of the frequencies awarded were transferred to support the second DFW-NRT trip, with the remainder put back into the unallocated pool.
(http://dmses.dot.gov/docimages/pdf82/34449_web.pdf)
Also, a side note... Jim, it appears someone was listening to you. SJC-LAS is planned to go away in October as well, being replaced with LAX-LAS (timed to connect to NRT). So, the beyond connections for the NRT-LAX flight just grew a bit. Not exactly as powerful as what ORD-HKG would offer, though.