I have experience going into very high altitudes in ketosis, and can say that being in ketosis was a huge advantage for me.
My ascents were in excess of 4000 metres, and to go where I was going, I needed to get a medical check up with clearance from doctors (I.e. Certified to do high altitude). That's where they put you on a treadmill, connected to electrodes, and read outs sent to 2 doctors who both have to agree you are ok to go.
The best way is to sleep overnight at a low high altitude, like 2000 or 2500 metres, and then proceed the next day. If you feel bad at 2000, then take it easy, stay another day at lower levels.
You will experience hypoxia (oxygen starvation) at ultra high altitude, but your body will compensate by making more red blood cells. This is why I eat a bit of liver every day for about a week or two before the climb. I make sure I have lots and lots of iron for my body to make hemoglobin (red blood cells). You will also be thirsty a lot (air very dry) which will deplete your electrolytes....so eat/drink lots of broth to restore electrolytes.
People who have a hard time coping at high elevations get massive headaches. If you do, take it easy. To releive the pain of the headache, you can take a bit of oxygen from a bottle...but, be careful. Just do maximum 15 minutes, and go to sleep / lie down and rest. Don't stay connected to breathing oxygen because you need to teach your body to cope and acclimatize. Oxygen will stop acclimatization. Only in an emergency do you want to continuously feed oxygen supplementation to a person at high altitude.
Dave Asprey got his inspiration for the fatty bullet proof coffee after seeing high altitude natives in Asia drinking fatty tea with yak butter and cream. Like "butter tea" in the Bhutan region. They are in ketosis and eat lots of fat in their diet. It says something when people who are indigenous to high altitude that they eat tons of fat when on high altitude climbs. Dominic Dagostino also did some research on high altitude and benefits of ketosis, too.
Hope this helps!