ejection seat bottle can't be used for normal breathing purposes as it has just about 10 minutes of air in it. The problem with F-22 pilot was lack of alternate supply and if OBOGS malfunctions or fails, you have to descend to below 10K altitude asap.If I am getting you correctly, pilot has option to manually switch on emergency oxygen supply.
What could be the reason for that F-22 fighter death then? He got late in decision making or he didn't had the one?
One suggestion - why not make the process auto i.e. if sensor sense less than adequate oxygen in cabin, switch to emergency oxygen.