Ian: The pro-biotics are unnecessary. If the topsoils can support life, it is already there. Even in a sand dune, you're depositing trillions of bacteria with every dump.
I wouldn't suggest carrying fertilizer. There's a fair bit of biologically available NPK (nitrogen, phosphorus, and potassium - the building blocks of DNA, proteins and enzymes) in poop and pee. If you were burying something without built-in fertilizer (paper only, clothing, gasoline in my work), then added fertilizer can make a big difference in kicking off development of a large bio mass.
Adding excess fertilizer would be a bad thing. Nitrates and nitrites are groundwater and surface water pollutants (they can lead to algae blooms in warm waters). Dissolved fertilizers can travel much further than bacteria - a well functioning septic leach field does all it's work with 2 feet of the perforated pipe, leaving no bacteria percolating deeper into the ground.
I was giving a pull-out-all-the-stops approach. Stuff I do to clean up a decades-old toxic waste site in months. For me, best backpacking practices would be:
Do it in a warm spot, mix with soil, add some water if easy. Those steps will result in many-fold faster degradation. And they weigh nothing.