Or you could dump the power into a realllly loud horn

i wonder how much heat you'd have to dump if you just used a resistive load and heatsinks. aluminum isn't very heavy. Look at the size of a rotor and brake shoe on modern cars, it can't be that much larger surface area as a heat sink to dump the charge. Although someone would have to test it. Since EV are so smart, wouldn't they know that all routes generally are downhill, and thus stop the charge at 80% or something? Further, the absorption phase means you have to dump excess current somewhere anyway if you're over 80% charged.

Interesting, it seems like keeping the battery at some level below 100% (e.g. 80%) would make sense then because I don't think regen would ever top it off that much right?

My understanding is that after 80% the rate of charge a lithium ion battery can accept diminishes.

Could the 80% Li-on battery accept the charge from the brake regen fast enough? Or would it still have to be drained somewhere else if the battery could not accept it?