I've been playing around more with ingesting PST files. I set up a normalization rule and command as Anthony describes below, and it appears to work perfectly until the verification step. I.e. when looking at the normalization report, it shows the extracted mbox folders, but then fails the verification step with "output file does not exist". I tried setting the Verification Command to None, but it still fails. I also tried an Extraction rule, on the off chance that it would then expect multiple rather than a single file, but still no luck.
Any ideas how to make this conversion from PST to MBOX work successfully on ingest? For now I'm just going to use readpst at the command line to convert to mbox before ingesting....