I posted a related idea in this forum that would help with PayPal, Authorize.Net and others. The problem with creating a custom action mentioned here is the documentation (or lack thereof) for building the DLL.
Another approach would to add a new action to OWS called "Reflection". The text (.net code) entered into this action wold be executed via .net reflection.
From what I've experienced, it is very difficult to integrate with any payment gateway that has a robust API. IMHO making it easy to integrate with a payment processor would be a major advantage to OWS and perhaps breathe new life to the project.