Ok, answered part of this myself. You can stop a node, move files around on the data disks, as long as they stay in the right keyspace directories, and all is fine.
Now, I have a single Data.db file which is 900GB and is compacted. The drive its on is only 1.5TB, so it can't anticompact at all. Is there anything I can do? The replication factor is 3, so one idea is to take down the node, blow away the huge file, adjust the token, and restart the node. At that point I'm not sure what to tell the new node or other nodes to do... do I need to run a repair, or a cleanup, or a loadbalance, or ... what?
It would be great to be able to fix a storage quota on a per-data-directory basis, to ensure that enough capacity is retained for anticompaction. Default 45% quota, adjustable for the brave.
My nodes have 5 disks and are using them separately as data disks. The usage on the disks is not uniform, and one is nearly full. Is there some way to manually balance the files across the disks? Pretty much anything done via nodetool incurs an anticompaction with obviously fails. system/ is not the problem, it's in my data's keyspace.