Since iOS 8, 'Allowing custom keyboards' refers to the ability to use other soft keyboards (i.e.: Swyft, etc) created by non-Apple developers. These 3rd party keyboards are a security risk as it can record your keystrokes and you have yo trust that the developer doesn't misuse information collected by the recorded keystroke data. My original post was regarding a need for an EXTERNAL keyboard where the dictation command can be enabled (either via keystroke or the appearance of a soft button the the screen. Right now, the only way to enable dictation is to disconnect the external keyboard and activate the soft keyboard. As a physician who dictates a lot of charts, I use a combination of typing an occasional sentence and dictating longer text as I'm a horrible typist.
Just a thought - does the "Fn Fn" command (pressing the Fn button twice) activate dictation similar to how it works on the Mac. I currently don't have an external keyboard to test out this theory.