I wouldn't race to use the super MC base.
-Herald of Khorne on chariot = 60mm round base (Large circle, just like all other models on/with Juggernauts)
-Herald of Slaanesh on chariot = 60mm round base (this is really an assumption, but since a chariot and two seeker mounts won't fit on a 40mm, its probably the best option)
-Herald of Tzeentch on chariot = Large Flying base (it's a Jetbike, thus this is the only option)
Can't be sure what people will accept, this isn't an extreme stretch for a 'counts as', but I really like the look of the 'right' beasts of burden pulling the chariots.