I love this idea. Stray dogs are present eveerywhere in the streets. However, you must find a way to train dogs to be loyal to multiple people and make them understand that they are escorts to whoever gives them food (I have no idea how to do this). Most stray dogs follow people in search for food only. From personal experience, they are very hard to adopt and after being fed will walk away.
Whenever you hear a true calling for help from any person, the tone of the voice is alot more urgent and naturally urges you to help them.
Maybe you could try to record some of these and program them into the whistle. Instead of a normal whistle which can be confused with anything, a loud cry for help connoted suffering and will attract the attention from anyone around instead of just patrolling officers.