I asked an experienced planter in Europe about this and here is what he had to say:
I haven't tried with Cuba-Hemianthus in brackish water. Please give it a try and let me know the results. I wouldn't be surprised if it could. It nature, you often see the type of plants growing in marsh areas (mix of freshwater and brackish water)
Since HC is relatively inexpensive, I think I'm going to give this a shot to see if it works.