Perry,
The 'cleanest' way I can think of to do this would be by creating a separate form made available outside of the Workflow webaccess, which generates appropriate XML data file and instead of submitting it via the workflow webaccess, create a custom CGI/ASP or whatever to initiate a new process instance. (Using, of course, the AWS API to create this.)
Does your user need feedback from the initiation? E.g. a cross reference number or process instance id? Maybe you could update another page in another frame with the feedback?
If you don't mind 'meddling' with the standard Webaccess, you can modify it (but after you have, Adobe will no longer support your version of it) to allow more than one submit type from one form - one to actually complete the current instance (and close the form, allowing the user to initiate another type of item to work with), and another that you would write the submission code to either generate a email to submit to the workflow agent, or to use another custom page to process the submitted data item. (This second suggestion I think would not only be harder to build, it would also be harder to maintain.)
The documentation to the API and the code for webaccess are your best learning tools for the API.
Hope this helps.
Sanna