Drupal 6 - Webform integration with Experian QAS (Address lookup)

Question

Hi, I'm looking to create a form solution for my client; but rather than code it completely from scratch I thought I'd try and hook into the Drupal Webform module. I think this will be fairly easy for the client to get to grips with as they are already administrating blocks, pages and other custom content types.

However, the big unknown is how to integrate Experian QAS into the forms. As far as I can see I need to:

1. Have standard "Address" fields in each form
2. Hook in QAS via a custom module to act on the relevant address field
3. QAS will reload the form adding in the extra data (form data needs to be saved?)
4. Form needs to submit, using some custom validation (Webform validation module)

I know there are additional processing and validation hooks available in webform, but does anyone have any experience of this or ideas on how to proceed?

I'm currently waiting on the client to get me the QAS source code so I can see how it all works.

Thanks!