...
Even simpler, why not try:
razor-report -M <mbox
Calling razor-report directly will work if your SA install only modifies headers, and does not modify message bodies (ie: report_safe 0).
In general Razor is not concerned with changes to message headers, except to the extent that they affect Razor's ability to parse mime sections. It is however sensitive to changes to the message body itself, and it's unclear if razor can deal with an encapsulated message generated by SAI believe the method directly above will not work because SA rewrites the mail headers.