I’ve been to a JASNA tea party/meeting, when they came here to Dayton. A few people dressed up in period Regency clothing, but most people wore normal modern clothing. Same with Sherlockians. Same with sf conventions, for that matter. The costumers and cosplayers do their thing, and everybody else does theirs.
JASNA’s not the SCA or Civil War reenactment. It’s reading papers and playing funny games.
Austenland is a totally made-up movie based on a totally made-up book. Made-up. Fictional.