You'll want to be sure to check your local laws before doing that. Most (maybe all?) US states require a working horn for safety reasons. Here, for example, is Minnesota: https://www.revisor.mn.gov/statutes/cite/169.68
I live in New Zealand, so no US state or federal laws apply (unless I get extradited?)
Yes, it is a legal requirement, which in practice could void an insurance claim, or a criminal case if I hurt someone, but sometimes you just have to make a choice.
> Every motor vehicle when operated upon a highway
At least if OP follows through on running his car through his key, he'll be safe. (How about using a relay attached to something that's powered during key-on?)
