Hmm "mydesign/content/datatype/view" will not work, at least it must be in the templates folder, i.e. mydesign/templates/content/datatype/view. This is working fine for me as I in "mydesign/templates/content/datatype/edit" has an customized "ezuser.tpl". Note the name of the customized datatype is still the same. I do not now if this is correct but it works:)
