Late to the party, but it is in the change forms. More specifically, it's an ACHR record (type = 1) in the change forms. Unfortunately, the change forms for ACHR records aren't documented at all on UESP.
Back when Skyrim was released, I released an alchemy tool like what you're wanting to do. I wanted to do what you're doing, but I ran into the same brick wall that you are - information about the inventory wasn't available. Now I'm working with FO4 doing something that similarly needs to read the inventory. What I've decided to do is place a stipulation that the items I'm looking for can NOT be in an actor's inventory (player or NPC) -- they can be in any other kind of inventory. With that stipulation, you can read the change records and figure out what the player has moved (thus assuming ownership) by reading the REFR records like you've already done. It's hackish, but it should work for your purposes, I believe.
Better for your purposes might be adding a special type of container via a mod, and have the player put their ingredients in it. Then you can search for your container's RefID (or BaseID), and read the REFR for it.