Hello BlackBeauty, and welcome to NarniaWeb!
Your project sounds like a very neat and fun idea! I'm sorry I can't help directly. But you might try PMing member LittleLionTamer. She has made the farewell dress and may be able to help.
I hope everything works out!

EDIT: I forgot to mention thewardrobedoor's page about this costume. If you haven't seen that yet, you should check it out! It's very in-depth and should offer some great insights.

Hello!
I saw this threat and I might actually be able to help you.
I used a pattern of Simplicity, to be exact I used this one: Simplicity 7417

But I worked to change the pattern quite a lot. First of all, don't use an elastic band for the underdress. It would look really weird if you want to add the wing to it. I simply added a couple of tucks and that did the job too!
You could also use a different pattern for the undergown if you feel like it. For the sleeves, I cut out squares and ruffled them.

For the blue gown, I cut the front part of the original pattern in two and added a little bit of "b**b curve", and of course, the pointy front.

If you have more questions, feel free to message me!

The thing I'm curious about is, you're going to be riding in the dress?

I would recommend making a muslin first, so that you can see how the skirt will drape over your saddles, and also maybe having the underdress be less of a dress and more of a shirt.

But a whole drill team sounds absolutely gorgeous! I really hope you'll post pictures if/when you do this!

Lovely idea! I would second Meltintalle's idea of making a muslin mock up first. Personally, I'd recommend either adding volume to the skirt or adding splits partially up the seam in the front (and maybe the back) to help you mount your horse better (and it would improve the draping).

I'm sure your whole team will look fantastic dressed up!

~Anna