I actually haven't seen that bit of fanon yet. I think it would be more fun to play with Zuko as Sozin and Katara as never-got-a-name-avatar bride, and then having an illict thing going on back then.

The one reference I've seen to Omashu was Katara telling Zuko the legand as a parallel to their relationship, and there's a point where she pauses, and he jumps ahead to 'and they lived happily ever after, right?" (which, in retrospect, was a bit out of character) and then Katara goes "ummm... yeah."