[TalnSG]

I did some searching, but it seems various sources for seaweed and similar plants in your area are process into suppliments rather than available as a comestible.

As for fillings, a few other things came to mind. Pickled herring (not with the tomatoes), mackeral and anchovies would all be suitable. Instead of the nori, maybe a cooked cabbage leaf would be a good compliment to the filling. Red cabbage would be quite decorative, if the filling went with the sweet & sour taste.

Of course your available horseradish is what most of the world uses in place of wasabi, so that flavoring will be very traditional.