I don't know if you will be going to the Magic Kingdom on your trip at all, but adults can make appointments at the Harmony Barber Shop - they don't do updos but you can get a haircut and they will put "pixie dust" in your hair at the end of your appointment if you request it. (Or pop by the "Sir Mickey's" shop in Fantasyland and ask for a pixie dusting from Cast Members there....)
I also know it isn't the same as a sash but you can also get celebration buttons either at Walt Disney World or on your
Disney Cruise....
I hope you have a wonderful trip, either way- it sounds like it is going to be a lot of fun to plan for and to look forward to in 2028!