I'd use the more common "surprise, we're pregnant, let's all be universally happy about it" party format as a template, and simply rework various divergent details/balloon colors appropriately.
In all seriousness, I'd focus on the guest list, and try to make sure you won't be pissing anyone off by preventing them from showering you with childhood anecdotes or photos carefully crafted or sourced over months prior, and of course, gifts you can exchange for cash.
Another issue to consider are flakes and no-shows – that is, people who will bow out of a cookout, who would fight God in his Heaven to attend your wedding. Er, reception.
Both of the above cases basically happened at my own wedding – we invited a bunch of people to a surprise birthday party for my mother, where the real surprise was that she was getting her most desired and most unexpected gift of all time. I had an awesome uncle who broke his hip skiing just a month before, and no matter how much pressure I carefully applied, I couldn't get him there without spilling the beans – and, unfortunately, tell one of my mother's siblings, and they all know within ten minutes… Anyway, I think he's still a bit peeved that he didn't get to come.
Overall, I'd say this: most people who elope or get married without a big to-do want the event to be about them, not all the weird family stuff. Give them this one day, and then give each other a lifetime without their meddling interference. It's a much better deal in the long run.