Hello Noddy,
The subject of the mail is in a mailParameters memo (XML) field of the mailing and this is the mailParameters / subject element.
As for any memo / XML field with Adobe Campaign, it can not be used in Query (Query, Enrichment, etc.) or Javascript (queryDef) activity.
On the other hand, it can be exported without any problem from the Extraction file activity:

So you can export this as a csv file and use it as a report.
This seems to be the only way we can extract subject using a workflow.
Note: This will export personalized fields as it is , so if you have used , something like :
<% = recipient.firstname%> then the Javascript code will be exported as such in the export file.
Regards,
Adhiyan