These might be advantageous in some ways: money savings, wider and easier availability, independence from pepper spray suppliers, international availability, more widely legal (though possibly not universally legal, unless used or described primarily as a condiment), no loss of pressure, etc.
They could be delivered using a mister that has a 'squirt' setting. A good quality squirt gun with fairly long range would be another possibility. Maybe there are some other good choices as well?
Spoke with a woman a couple of days ago who did something similar, for some dogs that had been harassing her on local rides. She used household ammonia in a mister bottle. She would put her feet up on the bars to get them out of the dogs' reach, and squirt them with the ammonia. The dogs didn't stop barking at her on subsequent rides, but they learned to keep their distance. So it worked well for her.
Ammonia, though, might not be the best choice for this. Apparently, it can cause permanent eye damage in humans. So it isn't clear that it wouldn't also do this in the case of dogs as well, in some cases at least.
Pepper spray might be a better choice.
So the question here, in this thread, is - How to make home-brewed pepper sprays?
I'm guessing that there are ways to do this, starting with hot chili powder, hot sauce, hot peppers, cayenne, or other starting ingredients. Would it be a matter of making a tea, then boiling it down and concentrating it? Surely there must be ways of doing this?
Another approach would be to buy some of the hottest cayenne or powdered peppers available, and just use it as a powder. Some cyclists have used this approach, though it isn't clear how effective it is. They carry the powder in a pouch on their handlebars, and just throw some in the dogs' faces. Not so good in certain sorts of windy conditions, perhaps; and the range seems limited; but maybe there are improved versions of this approach?
Pepper spray is illegal in some areas of the world, but I doubt that the same sorts of limits exist on hot sauces. They seem to be pretty widely accepted.
It would be interesting to refine the approach and the delivery systems, and to make this work.