Webform CiviCRM integration with REST
Author of the module here...

Great question. I've forseen a need for this and have over the past few versions been stripping out sql queries and BAO calls when possible and replacing them with api calls. But that work is only around half done. And the low-hanging fruit has been picked. The remaining BAO/DAO calls are mostly still in there because there is no equivalent api.

The reason for this approach is that CiviCRM supports api calls over REST, so once we decoupled Webform-CiviCRM from calling internal Civi functions, it would be theoretically possible to pipe all the api calls over REST (assuming you had a nice fast connection between the two servers).

The only "quick 'n easy" way forward I see would be to install CiviCRM on your host Drupal site just so the internal function calls won't bomb, then swap the wf_civicrm_api function in utils.inc to use REST. That should more-or-less work, depending on your use-case.

The "correct" solution is probably to write a CiviCRM extension, installable on the source site, that provides an api wrapper for all required internal functions, and patch Webform-CiviCRM to use that api instead of direct DAO/BAO calls. This would, in theory, allow you to host CiviCRM on WordPress and integrate with a Drupal webform.