We just discovered that the gorgeous dress from Johanna Mason's Official Capitol Portrait was designed by Dutch designer Jan Taminiau. The dress is from Taminiau's appropriately named F/W 2011 Collection, Nature Extends. It shouldn't surprise you to find out Lady Gaga has worn Taminiau's designs many times. Gaga seems to be a big inspiration for Catching Fire costume designer Trish Summerville.
Thanks to MyHungerGames.com for the info!