You can do this with Steem accounts if you're a power-user type and aren't shy of a command line interface. With the steemit.com GUI, it's no longer possible.
Changing one key will make your original master password not function for that particular use, so if you used something other than Steemit to change just your posting key, the password would no longer allow you to post/comment/vote.
Oh thanks... that's interesting. I'll get into more of the details of the CLI and APIs soon I hope.