Custom Render service question

This is kind of related to the post at "http://www.adobeforums.com/webx/.3bc480d1/9".

Would like to find out how to use a custom Render service to populate some fields on the form, at the "Initiator" step of a workflow app.

In the Form Augumenter, there's "Get Field Value" service, but there's no "Set Field Value".

There's a "Insert Workflow XFO" service, but it seems that it can only set those "AWS_xxxx" fields.

How do I set a field value (to non AWS_ field) in a custom Render?
(at the "Initiator" step of a workflow application of course)

I know that I can make a WebService call from the Form to populate fields on the form, but it seems neater to do the field population in the custom Render (if it's feasible).

thanks
James