What made this project successful was the promise of trading with other kids (this had a real purpose), the freedom to write about whatever was interesting about the period, and the hands-on aspect. We painted and burned the paper and then used wax seals.
I think this approach would be fun with any time period and a group of kids. Even better would be to actually send the letters (and maybe even packages) through the postal service.
I hope you have a wonderful 4th of July filled with food, explosives and gratitude for the liberties we enjoy.