The fancy rate limit logic of the journal that takes cgroup info and disk space into account is actually in systemd git master since a while.
And I don't see why timestamping log streams in userspace would be "slow", as the kernel automatically attaches a timestamp to all datagrams anyway, which is accessible via SO_TIMESTAMP/SO_TIMESTAMPNS. Basically on /dev/log the kernel attaches those timestamps it's just a matter of querying them from userspace.
And I don't think Neil Browns file system idea would work, since you'd need to a sane way to communicate the current write and read pointer into those log buffers to userspace (i.e. since we use it as a ring buffer, right?), but that would be necessarily racy, unless the files on those file systems get very very magic seek semantics at which point they'd probably be a lot more like a char device, than a normal file.