ATM, "Phase encoding direction" (dir) is a required field for fmap(epi). But it is relevant for other sequences (fmap, dwi) but is not available in BIDS specification for them. To overcome this specification people resort to list the value in a flexible and omni-present acq field:

$> datalad -c datalad.search.index-egrep-documenttype=all search path:.*_acq-ap.*
search(ok): /home/yoh/datalad/openfmri/ds000221/sub-010001/ses-02/func/sub-010001_ses-02_task-rest_acq-AP_run-01_bold.json (file)
...
search(ok): /home/yoh/datalad/openfmri/ds000244/sub-01/ses-00/func/sub-01_ses-00_task-ArchiSocial_acq-ap_bold.nii.gz (file)
...

So it seems that demand is there and I do not see a reason why dir should not be allowed to be used in those cases.