Ideally, all reward cards will be part of one of the deck sets and usable for those sets. So the current reward cards should be playable with any Chaos Legion rules.
I'm not sure if that will be allowed as soon as Modern/Wild is released, but it is something the team is aware of and would like to support in the near-future.
When comes to Guild Brawl fray. The Chaos fray, it does not allow to use Chaos rewards cards.
Is there any plan to add the Chaos reward card in the brawl or it will not be considered?
Without the Chaos reward card, the option to out a team is pretty limited as other frays.