I looked at fashion plates and consulted Costume In Detail and Nineteenth Century Fashion In Detail and finally decided to make a drawstring, lightly gathered neckline and smooth elbow length sleeves. I didn't want to make a plain white gown, though they are far and away the most common kind to find examples of. I ended up finding a fabric at Wal-Mart with a white ground and blue figures that was $2/yard. I did find quite a few original printed dresses and genre paintings of printed dresses, among them were these:
Portrait of Jane Ann Campbell 1820
Girl's Dress 1815
14 year old Mary Ann Gale painted in 1815
Cotton dress 1805-1810
I took Rebekah's regular dress pattern and made it up in scrap fabric and then drew the new, further back shoulder seams in magic marker. Then I drew and redrew the slanting back seams to get the placement and angle right. I'm used to the curving back seams that were popular in the 1860s, whereas Regency seams are more of a straight line. Then I tweaked the armscye, cutting it in deeply at the back to give the illusion of a "small back" that was popular in this era. I cut the bodice apart on the marker lines and redrew the pattern with it, adding seam allowances.
When making up the dress I lined the bodice in a light weight cotton and piped the back seams. There is a casing at the neckline with a tiny 1/8" twill tape drawstring and self fabric buttons closing the rest of the back.
She really seems to like it. :) She has a bonnet to go with it which is as of yet untrimmed, hence no pictures, but I'm working on it.