This one may be a little tricky because you would need to remember *NOT* to turn the head before you hit the "retro-key" button.
Perhaps a good solution would be have the code check for a keyframe occurence previous to the one you are attempting to place, then in the code, if there is none, have it set the pre-keyframe for the attribute you are attempting to set before it actually sets the one you are attempting to set.
"The Sage as an Astronomer: If you still see the stars as something above you, you lack the eye of knowledge." Friedrich Nietzsche