Friends of mine had their engagement party, all dressed normally, him in a suit, her in a pretty dress. Halfway through the night her niece felt unwell so my friend, her sister & niece disappeared for a little while (& me, I was their photographer). They reappeared & my friend was in her gorgeous wedding dress & shocked the entire group of people there when they realised what was happening. It was beautiful.
This is what a girl I work with did too. I think it is gorgeous!

My Uncle also did this - had a BBQ and half way through they went and got changed and came out dressed up - that was the first guests knew about it. Even their kids (second marriage for both) knew about it. My Aunt had made the girls beautiful dresses at night after they went to bed so it was a total surprise for them too! They did have people missing but looked at it that those who really cared for them were there.

Personally, I think it's a great idea. We had wanted to elope or do something similar as opposed to the massive hooha that traditional weddings can be. Do whatever feels right for you!