Easiest way without damage is to wrap vice grip jaws with tape(or use a piece of cloth) Clamp down on stud where allen bolt holds bracket. And then just turn face counterclockwise. Did it just few minutes ago. Just dont clamp down hard,just enough to hold it in place.
I took mine to a jeweler and he said that if they take the back off they could not garauntee that it would remain water proof. The cost to remove the back and replace the battery (they wouldd have to send it out) was about $50.00.
Good post revisit timing, just got a HD clock for my Road King - only problem is I have the chrome controls which have the chunckier mounts and the screws go down into holes (hence the bracket won't fit) - might have to get creative with some washers!