incremental backup needs to apply diffs to baseline backup, for prepare. It can't apply diffs to compressed files (.qp, .gz, or anything else), without decompression. If you were on Windows, you could use a transparently compressed directory for base, and/or incremental directories. Maybe you can have some luck with ZFS on Linux, according to google it can have transparently compressed directories, too.

Thanks Vlad. This workaround might help some people, I don't really need it, I'd like to leave this feature request standing!

The old --compress option compresses file-by-file and automatically decompresses for the diff for incremental backups. Only it uses QuickLZ, and current gzip compression has ~40% smaller results! Having some programming background I think it's possible with a small workload to incorporate gzip into mariabackup the same way it uses QuickLZ.