Perhaps not the perfect solution, but I have found one! And it works for me.
I'm breaking some SAP rules here. Need to see what the ramifications are...
I can enter SWWIHEAD and change a workitem record to 'ready' from 'completed'. (The standard funtion will not allow this, hence forcing it with the debugger.) Forward the workitem to the user who needs to be able to make attachment/data change in the form.
User completes workitem and then all stati go back to completed.
Case management updates, and report we built (reading data from case mgmt) shows correct data and attachments.
Ideally I need to configure a change workflow to grab that old form based on a selection parameter entered by user. Then they can change themselves without developer intervention.