H500, I can't imagine who told you that Halon is so toxic it will kill you if you use it in a confined space. Phooey. Utter hokum.
CO2 might displace breatheable air if you use enough of it in a closed box, but no cockpit is that enclosed or airtight, and I doubt very much that the extinguisher is anywhere close to holding enough of the stuff either. And as said above, you will choke on the dust from a dry powder.
Halon is there for a reason. It is BY FAR the most effective extinguishant around, by orders of magnitude. Be thankful it works - the alternatives (there are none that work well) don't bear thinking about.