The XML piece is solved. Microsoft SQL has specific features for importing XML, so the hardest part was learning the correct syntax.
My current problem is automating the process of walking the inbox to export the attachments on new messages.
As it turns out, the ideal tool needs to capture message metadata as well as the attachment, as these values can also be significant for some messages sources. The additional data to be captured is:
The message From address
The subject text, which should be in form: Report Domain: example.com Submitter: google.com Report-ID: <random-unique-string>
I suppose solving this part of the problem would require creating two messages from every email, the attachment file itself and a text file with the metadata and a pointer to the attachment file. Then the import process becomes more complicated as well.