Since the auto-complete option is now in forms and surveys it would be a welcome addition to the custom fields section also. For example, a contractor doing a job at a different address than the clients' address. So now with this proposed feature, a client would be able to use the auto-complete twice on the form(once for his/her address and another address for the job site(custom field)).
We often have several addresses inside the contact record and having the client be able to use the auto-complete while mapping to the correct custom field would be GREAT!