In some markets, BMW sells a pedestrian warning option - below a certain speed, it's essentially a noise maker to warn pedestrians. They did not see fit to offer that for the US market, but the parts exist. I do not know how much it may cost, or how hard it would be to retrofit (even if the dealers could do it - often, they can't get parts that aren't officially released in a country), or what it might cost installed. It doesn't seem to be a very expensive factory option, but installed there verses later, the price on both the parts and labor could be significant. It is tied into the computer so that it knows when to turn on and off, so it's more than just a simple plug in (unless, if you're really lucky, the computer recognizes it's there on the computer buss). IOW, at least in some places, there is a factory solution to the pedestrian warning (but not the horn!).
To save weight, the wiring harness plugs probably do not exist in the USA wiring harnesses, so integrating it may also mean changing a major portion of the wiring...a formidable endeavor!