I've just had a cucumber to stop my tummy rumbling. I've been doing this a lot lately at midday--usually cherry tomatoes or a cucumber. I'm not sure it's the best strategy, but it seems to tide me over until I have a miso soup, then dinner.
I've noticed a couple of people (GMH and rawkaren, if I'm not mistaken) have lunch as their main meal on fast days. I'm thinking about giving this a try.