You don't need nitrous oxide. All you need to do is displace oxygen with an inert gas like nitrogen. A scuba rebreather can malfunction in this way, and it leads to painless and unexpected unconsciousness. (That's why they have multiple oxygen sensors, and why you replace them on schedule if you're smart.)
You would not want CO2 in the execution process, as the feeling of asphyxiation is caused by inability to dump CO2, not a lack of O2. If you can keep exhaling the CO2 that you produce into a large volume of an inert gas, you'll pass out without discomfort.
All you need for effective and humane executions is nitrogen. There is no medical mystery about how to do the job. There is no need for drug cocktails and contraptions. There *is* a need for some new legislation, or court rulings. If we're going to keep executing people, we can at least stop torturing them to death.
According to wikipedia, Oklahoma has made inert gas asphyxiation legal, but the ABC news citation link is busted and I can't find a primary source.