This is an interesting question -- You don't mention which version of SharePoint / InfoPath you are using?
I went and took a look at SharePoint web services, to see if there was a way to start a workfow or get data from one via web services (I'm always surprised by just how much you can do with those in InfoPath), but I didn't see anythign off hand.
Workflow Methods looks like what you might need for writing code for this.....