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I had a quick look and I think I found what you are asking. I haven't tried it but this should give you what you are looking for if I understood correctly.
I think you will have to give that Datum Tag to a user function AskFeatureOfDatum from which you will get a Tag of a DatumPlaneFeature. You can use the datum feature for the builder:
datumPlaneBuilder1 = workPart.Features.CreateDatumPlaneBuilder(datumfeature);
plane1.GetGeomtry() - this will give you NxObject where one is the face used to create that datum.
Hope it works.