I've tried several types of brushes on these Emerica Romero 1s or 2s I found at Savers that were in almost brand new condition, save for some strange white specks on a few spots of the upper and apparently some ancient calcified dog dookie in the grip holes. No dice. I suppose I'll either try a brand new toothbrush with a firm handle, or resign to using plastic toothpicks since nothing else has worked.
Thanks everyone, looks like I tried the right things initially and am just dealing with some reallll old gunk. It's those Leo shoes with the 2 white stripes on 'em, so they're probably quite old.