The --Istdout option outputs a hex dump of the IDENTIFY DEVICE data (512 bytes). An option to output that data in binary instead would be useful.
Related to that, it's not possible to output DCO identify data in either hex or binary. Options to output the DCO data in hex or binary would be good to have. (Actually, if you do hdparm --verbose --dco-identify you do get a hex dump. But it's not in a very friendly format.)