+1 The anon rule is mostly about the writing period. You could be what you think is a total stranger to them and they could still guess you were their author after reading the fic. That's out of your control. Keep mum while you're writing and then enjoy the (hopefully private) OMG IT WAS YOU convo once they finish reading the thing.

(Also - they may end up being upset if you write them something generic when you knew what they wanted in their heart of hearts. I really doubt 7 days worth of not knowing its you is more important to them than getting a fic they really want, especially after they had 3 months of you keeping it a secret.)