awwwwwwwwwww that sounds so sweet, my GF totally surprised me when she asked me to marry her, we were in bed about to go to sleep when she handed me a ring and asked me to marry her, so I say go for the totally unexpected time, makes it more special I think, and I think the plan you have is excellent!!!
good luck!!

I agree with d. you really can't go wrong with the element of surprise! my cousin and her fiance planned their engagement and she designed the ring, which was being custom made in miami. they vacationed there together to pick up the ring and he proposed to her on the beach, which was very romantic and everything... but when she was telling me their plans i couldnt help but feel a little disappointed for her. if it were me i would like to be totally and completely blindsighted by it, and your idea sounds perfect. i know if i opened the door and faced a proposal like that i would be stoked.
if you need an idea for what you could say to get her to the door without her thinking its you, try scaring her with something like
"your front yard is on fire!" (not that cuz its lame) but something that will REALLY freak her out and make her panic and RUN to the door. good luck! and let us know what you come up with and how it goes!!!!