I hope others can answer your question regarding "dry" fertilizers that can break down and penetrate through already-applied mulch. In the meantime, I would suggest you consider foliar-feeding your plants using water-based fertilizers. You'll find a number of these available at your garden center (or Lowe's or Home Depot) or you can look on the Web for non-synthetic fertilizers. I used to get an "organic" one via mail order that was a dark brown and smelled like my children's baby vitamins (from the 1970's) and it worked really well. Just follow the directions for strength and timing.