You should gradually build testimonial pages for cities where you have had happy clients. Ask for a photo as well to add some variety (and an entry in Google Images fwiw). Give it some thought as to their WIIFM factor, e.g. mentioning their business gives them exposure (don't offer a link).
It's not possible to rank for any random place name. I see a similar situation with SEOs who want to offer global services. They put some city names in the title tag, but you can't fit more than 3-4 names there, but I have seen some success for them in the regular Google SERPs, but not for Google+ Local.