We found a solution for this and it's far from perfect. You have to create an "Update CDO from Form Data" processing step and map your fields. This appears to give you a "handle" to the object; to update ONE field to a static value, you have to insert another "Update CDO from Custom Value" step *immediately* after that to set your field to the desired value. Order is very important. This appears to work.
However, there is an issue. You cannot change the order of the processing steps. If you somehow get them out of order, you have to delete them and re-do them in order. Eloqua has promised this to be fixed in a future release...
Hope this helps someone else!
Todd
