I think when the scanned document is dropped in the watched folder you will need the metadata put there as well. May be you can ask the file name to have blahblahblah_taskid.pdf as the naming convention or the xxxx.pdf and xxxx.txt files are dropped there and the .txt file has the task id and your watch folder is looking for .txt files as the trigger.
The other way I've solved similar problem is through email endpoint. Just before the task creation you can send an email with process id and other info in subject line. The user either completes the task via option a or he replies to the email with attachment. Then the email endpoint will pick up the email and you'll get the process id from subject which will allow you to find running task. Then after use the links that these guys posted to create the complete task call.