i think it's a great idea! if someone had to travel & said they couldn't make it, you could always let them know if you wanted to make sure they were there - a friend of mine was going to a friend's 'engagement' party that was really her wedding but she told a couple of close friends who had to travel & were going to save the trip for the actual wedding.
Thats what happened to my cousin... when its a 6 hr drive away, and you know you have to do it soon for the "wedding", going a few months beforehand for a "birthday" was out of the question for most... not a matter of "we couldnt be bothered" at all!
So she did regret missing so many people, and had wished she had at least told certain people so they would be there... A lot of my family is out of town so this would never work for us, also because Ad's dad is a celebrant, so would have to know anyway!!!

But since PZ said she didnt have out of towners, I think its perfect.... I want to see lots of photo's!!!!!!