Just for general info, halon is used on airliners on the engines, in the
cargo holds and in the extinguishers in the cabins.  It's used simply
because its the most effective agent they've found for starving a fire of
oxygen.

As for venitlation in the cowl defeating the halon, that would be true of
just about any fire agent if you didn't first remove the fuel from the fire.
In airliners, we pull the fire handle before shooting the halon into the
cowl,which shuts off the fuel (among other things). My first step in dealing
with an engine fire in my plane would be pulling the mixture to cutoff, then
pull my fire handle.  If something else is feeding the flames, it just isn't
your day.

Also, not buying halon because it isn't plentiful for recharging an expended
bottle is silly.  If you can't get it recharged, buy a new one.  Besides,
how often does anyone expect to be using one in the first place?

Skip Slater