Has anyone considered simply asking the patient to sign off on these things as well? I realize many wouldn't but at least some would.

In the US, HIPAA gives patients a right to access and have corrections added to their medical record.

But in my conversations with a person I know who does this work -- I don't think that the typical problems with patient charts are anything that would be remotely noticeable to a patient -- they're often deficiencies of a technical and/or clinical significance.