Bromoform & co. is rather volatile and toxic. The nice feature about seaweed in this case is it has bromoperoxidase pathways, so the halomethanes are produced in low concentrations and bound in (I presume) cell membranes until digested, releasing it right where it's needed, in the fermenting stomach.

You'd have to engineer a digestible slow release halomethane source. Or just...grow it.

How hard can it be to scale seaweed farming?