Great question and topic. I wouldn't think that the gassing is probably anaerobic, but obviously, you are going to be able to tell more than I can.
Biochar is porous by nature, thereby letting lots of air in. Its porosity is one of its greatest attributes. It allows drainage to improve greatly in areas with heavy clay soil like mine. It also allows the biochar to absorb 6 times its volume of
water through the tiny pores. The tiny air pockets make sure that the
roots of plants can perform their crucial oxygen exchange that they need to do.
I do think you're smart to put castings, nutrition, etc. with the char. Your customers' results will be better in the long run, and they will recommend your product to others. However, I think you're right. An air valve would be a good idea. Thoroughly mixing the nutrition with the char would make sure that there are little pores of air throughout, but I'm sure you've already thought of that. Sounds like a great business, and I wish you much success.
John S
PDX OR