I didn't really address your basic question -- is the method too transparent? I haven't tried it in front of an audience, but no, I don't think so. Remember, the audience sees the backs of the cards in the deck and they appear to be free of ny kind of writing. Also, the packet add-on is covered very nicely. I don't think there's much to worry about.
I'll admit, though, I was considering memorizing the cards as well. This shouldn't be too difficult -- I've memorized a whole deck (Tamariz Stack) before. And with this, several different cards map to the same letter, so it makes things a bit easier.
Another idea, though I don't know how practical it would be -- marking the faces of the cards. Perhaps a very light pencil mark in the index corners, or something like that? I don't know. It's just a thought.