Only if you have 2 Rigard and you levelled the costume for each one (to get the bonus also for the original) → Is it possible to use a costume version of a hero separately in Alliance Wars?
Since most players have dupes of important 4* heroes, is not so hard (if you have feeders in TC 11 like me, for example). Took me 10 minutes to level Li Xiu costume and still remained food. Much faster and cheaper levelling a costume. I always keep the feeders in TC 11 for emergencies (an unexpected hotm, a 2* tournament or something like this).