I think your email should just confirm that you want to remain on waitlist/considered for RD. Your longer waitlist letter describing why you're awesome should wait until after January 6th when they start reviewing everything.
And just to add, yes, they are quite mean about not following directions! I called the admissions office yesterday and when I asked if I should call my waitlist manager they said "wait until after Jan 6th, like the instructions said". They were a bit unpleasant but luckily I didn't give them my name =)