Wow, this is really a crazy idea. I like it. I wouldn't worry about sugars or tannins. As you know, the native bacteria act pretty quickly. I think the main goal would be to knock back the bacteria and dry as quickly as possible. How about this: At the end of the sparge, add some more water, boil, drain, and oven dry at a fairly low temp - you don't want roasted draff, presumably. Once it's dried, just store like grain.