With the newest update, there is a new option along with creating a lockscreen password, which will allow you to add in a message for the person
Of course, as you'll notice, there are two times when this new feature won't help at all. First, if you're setting up this new option for a tablet, adding a phone number would be unhelpful, but you could always set the message to the address of a meeting place and time in hopes someone has found your device. Second and most obvious, if your device has been stolen, none of this will help, because it seems quite unlikely that a thief would call you to return your device.
Still, it's a nice addition to the service. The update should be rolling out to everyone within the next week or two, as is the usual timeline for Google App updates.
source: Android Police