I didn't think I would choose to do this either, but its worth lots of points.
The prequel would take place 6 years before the book, and Bernie and Ryan would be in the full swing of infertility and trying to concieve. Toward the middle of the book they would get pregnant with Emma (who in Chasing Rainbows we find out is born with a defect and passes away 5 days after she is born). We would follow the characters through that, and through the beginning of the demise of their marriage (in CR they get divorced). The "Happy Ending" would be around Bernie's father (who dies in CR) and how he supports her through it.
Maybe it would be a really really depressing book, but I think it would be interesting to see the writer's perspective of this turning point in their lives. After all that is the event that CR focuses around, and it was 5 years in the past.
I am married to my best friend, my soulmate, my fence post.