Accessing Drupal 6 data using Drupal 7 entities
As I understand it you're discounting the Remote Entity API module because you don't have a RESTful service...
The module actually relies on the Web Service Clients module, which provides that REST service for you:

Parallel to the Services module, the Clients module provides the ability to implement pluggable clients to external web services, including external Drupal sites running the Services module, the main use case being connecting to other Drupal installs via the Services module with either the REST or XMLRPC server.
Clients module allows you to create connections to remote services with an admin UI. These handle fetching a result from a remote service. Different types of connection are provided for different types of services. There is a Drupal client connection type for different versions of Services (5.x-0.92, 6.x-2.x, 7.x-3.x), hence Clients can be used to connect to sites with different versions of Drupal.

Emphasis mine.