it's me foja, i talk to myself all day so might as well answer myself..
you can create a customized graphic document in photoshop by choosing > FILE. OPEN AS, then choose RAW. however, ther is no option to choose 12 bit (or 4 bit per colour?) depth.
i soon realised that a plain PNG should be ok, otherwise someone would have said otherwise.
so carry on up the undefined, virtual and degrading digital path to you uncertain success.