It seems like it has to be a software issue, which makes it even more perplexing that Honda hasn't solved it. The reason I think it's a software issue is because even with an empty battery the engine can provide approximate 100 horsepower via the generator, which although that's very weak for a 4,000 lb car it should still at least be functional. Also I have never heard of someone in this situation having no power, i.e the car comes to a stop and will not move, instead all of the reports that I have ever read including yours the speed drops below 20 for example, or 7 in your case going uphill, but there is still power. Just to confirm, on the power display your needle was exactly at the 270 degree mark? In normal driving when the needle is above that mark there is power, below that mark it is doing regen, and exactly on the mark you are coasting. In normal driving if you take your foot off the pedal the needle will drop below center because of regen, until you get below about 5 mph then regen stops and the needle goes to center just prior to coming to a final stop. Also while coming to a stop sometimes I want to coast farther so I press the accelerator pedal slightly to stop the regen, if I hold the needle at center then it is coasting almost like being in neutral. Anyway do you remember if the needle was exactly at 270 or was it maybe slightly above?
Either way it sounds like when this problem occurs the car has gone into some type of limp mode, which a lot of cars do when there is a major problem that can lead to damage to the powertrain, but they give you enough power to at least move off the road and get to safety. My theory is that a software glitch causes the system to mistakenly think there is a problem that requires limp mode. But unfortunately this software glitch doesn't seem to leave any codes in the computer for a technician to view later, which is also why people with the problem don't seem to get any warning messages on the display. Also turning the car off and on again usually fixes it so that seems to prove that there is no actual problem with the powertrain. Although your latest experience is less common where turning the car off and on again didn't solve it, even temporarily.
One thing you might try if it happens again is switching to HV Charge mode. I say this only because people have reported that HV Charge works differently than normal HV, it's more than just the expected extra RPM's to charge the battery, but other slightly different behaviors, to the point that some people drive in HV Charge all the time instead of HV. There is a slight possibility that the software glitch does not occur in HV Charge, not by design, but assuming the software glitch is a typical software bug where an oversight in the programming causes problems in certain exact scenarios, maybe by some luck that buggy piece of code is not in the HV Charge routine.
Also if you ever accidentally let the EV miles get to 0 (which is easy to do) you can use HV Charge to get just over 50% charge on the battery. Of course ideally you would realize that you are in this situation when you are still far enough away from the mountain that HV Charge has enough time to recharge the battery.