Rihanna has the best leg in the industry and considering it's not a huge nigjt for her she can go for a short dress. She could wear something like she had at VMA 2007 or BBMA 2006 but more mature.
Barbados doesn't really have a traditional attire expect for Crop Over outfits which can't be worn on a red carpet
And Rih should NOT wear a short dress. It isn't elegant enough and I don't want people focusing on her legs during her urban contemporary album acceptance speech