Here I go reviving a dead thread (yay zombies!) but I've run into this same topic recently, and wanted to share what I learned.
Saving a form as Draft within Workspace results in a write to the database. That's mostly obvious, it has to be persisted somewhere. The problem is exactly WHERE in the database, and WHAT can you do about "old" drafts.
WHERE is the Task table, tb_tasks, and the Form Data table, tb_form_data. You can find all of the draft forms in your system with a query like this:
SELECT TT.instructions, TT.task_description, TT.step_name, TT.service_title, TF.initformvariable, TP.canonicalname, TP.commonname, TT.start_time, TT.process_instance_id
FROM tb_task TT, tb_form_data TF, edcprincipalentity TP, tb_assignment TA, tb_queue TQ
where TT.id = TF.task_id AND TT.status in (2, 4) and TT.current_assignment_id = TA.id and TQ.workflow_principal_id = TP.id AND TQ.id = TA.queue_id
Note that digging around like this is not recommended at all! At least this is just a query, not a DB change, but there's no guarantees that indices are properly used and running this query won't consume your resources!
The draft form is in the TF.initformvariable. The rest are there to be informative to the reader. The TF.initformvariable column equates to a LiveCycle Document data type. You can't really hand edit it or anything.
WHAT you can really do with this is, unfortunately, not much directly. The good news is that as a Task item, old drafts should I believe get cleaned up using the Adobe Purge Utility, or the Avoka Purge Utility for a bit of a plug.