Chia seeds sounds like a good compromise. You can also make a sort of goopy pudding with flax seeds. Another thing--I wouldn't worry too much about the "purity" of a fast. It really isn't established that absolutely no food is better than a tiny bit of food, Valter Longo has established that short periods of a fasting mimicking diet can have a strong beneficial effect. People will argue that only water-only is true fasting, but our metabolism doesn't work by semantics, there's an awful lot of territory between I can't believe I ate that whole pizza and I skipped a few meals, a bit of chia will still leave you way closer to that water fasted stated.