I've found the metadata option in the export panel where the default option is "create sidecar file". I can switch it to "none", but "embed in output file" is greyed out for MXF.
Also, here's a broader question: How can I write tape name data to the source files? In FCP 7, when you change the reel name of a supported clip (like MOV), it warns you about modifying the source file on disk. In Premiere, the tape name seems to be saved only in the project file.