This is an incredibly sweet idea. "Open when you need to know how much I love you; open when you don't feel beautiful; open when you need a laugh; open when you miss me; open when you're mad at me; open when you need a date night, etc.
Yes, it is a wonderful and very romantic idea. "Men ~ take note. Very romantic idea. If you don't know if she has the dress you want her to wear, buy her one and make sure it's a stretchy fabric so that you have a better chance at getting her right size.