The active anti-methane compound from seaweed is likely bromoform:

https://link.springer.com/article/10.1007/s10811-016-0830-7

It would be a cheap food supplement since the methane inhibition required only about 2% seaweed in cattle food, which in turn implies a bromoform feed content of only about 35 parts per million. Though no doubt some farmers would prefer to be able to say that they add seaweed rather than a particular chemical to their cattle feed.

of course nothing is free but they think it may be feasible: https://www.symbrosiasolutions.com/team