I discovered several things that needed changing to make it work:
- Time Machine now prefers APFS
- It won't work with a zvol that you format as APFS Encrypted within Disk Utility. This is what gives the bogus "zero KB available" message, apparently because Disk Utility uses a different method of creating the encrypted container than the wiki's manual method.
- But the manual method of encrypting an existing formatted volume currently in the wiki doesn't work with APFS. You have to say "sudo diskutil apfs encryptvolume [Disk ID] -user disk" instead. It takes the passphrase interactively, not on the command line, and it does a background conversion in place, which is apparently how Time Machine likes it.
Here's a bonus tip: at the command line, say "open /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/GenericTimeMachineDiskIcon.icns", select the first icon in the resulting window, ⌘-C, then say Get Info on the disk icon and ⌘-V the copied PNG to the disk to "bless" it as a Time Machine disk. For some reason, Time Machine won't do that by itself, as it does with normal external volumes when it first starts using them for Time Machine.