Hi PotPourri,
It'll likely require some code, but its doable. The other issue is that it will require two operations - submit to the library and then post to the additional column using code. You'll have to submit and then query to get ID and use some SharePoint mechanism (WebDav?) to write to column. I haven't done this before, but in theory it's doable. It will, however, require some coding.
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra