I use to tan a bunch of fur and have made both breaking and cleaning size tumblers (7 foot diameter types to the barrles you speak of). Your drying capcity will be limited by the water absorption capacity of the sawdust (not much I'm affraid). In the big tumblers during warm times and low humidity you can get the fur dried fairly fast; there is a lot of air movement in the tumbler and exchange with the atmosphere. I'm affraid in a drum, your sawdust will suck up all the water it can and the furs will still be quite wet. You may also find that these semi dry furs are hard to free of sawdust.
Also, tumblers, at least those I'm familar with, do not have a "through axel; You may run into problems with your furs gettinig tangled on the axel. My drum rides on wheels which are powered. The rotating wheels roll the barrel (like a big rock tumbler). Inside there are dividers which lift the furs and drop them through the center of the drum. My large unit has axels which are fixed to the sides (ends) of the drum and is driven through the axecl via a chain.
If I can help let me know.