Some do have word clock. My Motu828mkII does.
But if you are talking running multiple Firewire audio streams (Motu says I can daisy-chain 3 of my 828mkII's for 60 inputs) then that is probably all handled by the firewire drivers they have written.
If you are very serious about wanting to chain devices, I wouldn't waste my time posting on general boards.... you should contact the engineers of the particular interface you are wanting to purchase before you go down a road and find out there isn't a way to do what you ultimately want!