This would only theoretically work if:
The mane 6 were replaced by some other ponies that did not know twilight the way the current characters do.
Twilight had proven to have the knowledge and magical ability of Celestsia
Otherwise, people writing to Twi would be really odd to say the least.
Speaking of letters, though.. Since Twi is a princess, she doesn't need to write letters to Celestia anymore, right? So, I think we'll see some cool new ways to explain the morals in future episodes.