First, I'd like to say that I really appreciate all the feedback.
While the "immediate" message may be the only viable option, I've still got about 2 months before this would take place, so I may leave this up and try to see if anyone comes up with a solution to the original request. We don't actually have the atari set up or anything, and part of the surprise would be her actually getting to play crystal castles for the first time at our place.
As it gets closer to deadline I may have to re-evaluate, but for right now I'm gonna hold out hope that someone might be able to program a message into the actual game, even if it's just a short one. Please feel free to keep the suggestions coming, this is great!