Nothing points to 姿 being read as し rather than すがた here.
Affixing -姿 to clothes etc. is very common and can be done 
with lots of words. "洋服姿" for example gets 6 times the hits 
of "民族衣装姿" on ngram.　"ジーンズ姿" gets 19 times as many 
hits.

http://ejje.weblio.jp/sentence/content/民族衣装姿

from the jlpt1/2 book again. the first four characters are in the dictionary already.