Don't know about other fillers, but cream of wheat is a tried and true filler that has been used for a long time.
You don't want to compress it much, but you do have to compress it slightly to keep it from mixing with the powder.
Yes, it will get compacted. I know because I had to dig some out of cases that I pulled down. But even compacted, as long as it is dry, it turns into a fine powder when fired...wheat flour I guess...
Moisture could be a problem. I don't leave cases filled with cream of wheat sitting around a long time. And my ammo crates have desiccant.
I even use it with Trail Boss in some "bunny fart" loads.
