While wardriving and warwalking aren't illegal (as long as you don't actually connect to the network) I think most people you approach this way will see you more as someone that isn't particularly honorable. There are many stories of people who discover a security issue with a network and then tell the network administrator about it only to then be talking to law enforcement not long after.

You would certainly be identifying people who could use your services, but be careful with it. I'm not sure how the approach would go over with most people.