I have a grain mill as well and have added things like ground soy beans, groud rice, oats, chickpeas, ect.... They are very nice, the soy and brown rice soap had a subtle exfoliating that was not too irritating.
I do HP and add everything after cooking, so it is not a matter of lye penetration as the only means of preventing mould growth in my opinion, as lye will digest anything organic including your goodies, so is not nessicarly a good thing.
Most mould and bacteria need water, oxygen (except anaerobic micros), and available nutrients for growth - so removing one of these factors will greatly reduce the chance of mould/bacteria growing on your bits. Most dried produce can be added to soap with no issue (like whole oats) as they are too dry to grow anything, as long as they do not get wet on the outside of the soap where they are exposed to the oxygen and not surrounded by soap. Soap is a salt of an oil and most water in soap is not available for growth, especially with discounted water and if dried in a well ventilated area.
Where most mould and bacterial growth occur is when things like fresh flower petals, fresh fruit, fresh leaves where they are wet on the inside and start to decay and mould, as well as when people package or store soap with organic bits on the out side of the soap in a closed container and condensation can collect on the organic bits and growth can occur. I would say as long as the item is dried and does not go bad on it's own (regardless of the soap), it would likely be fine in soap, just be sure to keep everything dry and well ventilated while the soap dries so there is no condensation.
I also noticed that you were considering milling nuts? I would check with your mill that you can grind nuts and other oily/wet items, as mills like the Wonder Mill, Kitchen Tec and Nutrimill cannot be used with oily items as they will clog and may damage the mill. Just though I would let you know as this is the case with my mill.
Hope that helps! There is a lot of possibilities out there!
