I think this is a great idea, and it's one that some cultures have "built in" to their baby-related social customs.
The 100th Day parties is sort of a "coming out" party for the baby, and it used to be a celebration of baby surviving the neonatal period. We make it into a birthday party of sorts, with lots of friends, food, treats, cake, etc.
Some people bring small gifts, but we advertise it such that most do not bring anything. It's ultimately a celebration of the baby's birth and health, so it's nice to keep it simple and fun. This can be great for people who want to wait for the gender. It their idea is to wait until the baby was born because they chose to not find out the gender of the baby before birth, but I think your idea is similar and would work!
A sip-and-see is great since it give you the option to host a normal celebration.
You may have heard about them before and not even realized it. Some people call them a baby meet and greet. If you have ever been invited to one you know how they work. Everyone is able to see the baby and play with her and the mother gets a little break for the afternoon as everyone wants to hold the baby, change her and feed her. Expect the mother to share her birth story with you and just a little finger food. Some people might bring gifts but no-one should fell obligated.
If you are like me you are definitely leaning towards a meet-the-baby BBQ instead of a baby shower with cheesy games.