Is it an otherwise working barn which will be cleaned out for the occasion - it which case it will still be filthy - or is always used for events and just has a rustic feel - in which case it will be cleaner?!
How formal an event is it?
If possible I'd go trousers, but nothing wide leg or anything touching the floor. As flat shoes as possible, wedges maybe. Dark colour on bottom half at least. Bring the wedding style from waist up. Sparkly/sequin satin/silk top. Great hair do, with or without decoration, big earrings, glam make up.
If its later in the year and going to be colder, I'd invest more effort in the jacket than the top underneath as it will be worn most of the time. Just looking at Zara, they already have tons of jackets in the blazer section, from chanel boucle styles, leopard, zebra print, velvet, cropped, double breasted. Pick your style, then wear a plain silky shell top underneath.
Nothing dry clean!