Alternatively I will try to use either mri_label2vol or mri_binarize with --match 17 or --match 53 to try to export either an ordinary or binary volume from the aseg.mgz-file.

mri_binarize --i aseg.mgz --o left_hippocampus.mgz --match 17

Do you think this approach can work? I guess I will have to convert the .mgz files in to .gipl.gz afterwards?