I think your worry is justified. This isn't like Santa Claus and the Tooth Fairy, where you are simply a passive supporter of a well meaning lie. You are thinking of actively deceiving your daughter.
It might work if you make it very easy for her to figure out that it is you "behind" the box, and that you never meant for her not to know. It's a little like role play, where people pretend to be someone other than themselves in order to work out problems and experience situations that they don't want to face directly.
If you implement your box this way, it is like you are giving her permission to express her anger or frustrations, and promising her that there will be no repercussions. It would also give you time to think about things before you respond to what might potentially be an emotionally charged situation.
It's an interesting idea. Don't expect it to have any deeply meaningful purpose, but I don't see any harm in playing "magic box", as long as you keep the explanation so fanciful that when she gets a little older she won't have any trouble figuring it out. Maybe you could discuss with her what sort of magical creatures might be in the box. Fairies, maybe, or trolls (fairies give nice answers, trolls are grumpy). Show an interest in what comes out of the box. Give her the opportunity to share the message with you. Share her story. Exercise her imagination.
But don't, please, pretend to be a "real" friend in a deceitful way. It is similar in practice, though I'm sure not in intent, of adults who pose as children in order to befriend children on the internet. Some of them may have good motives, but I don't think such deceit is ever a good idea.