I have a critical database that needs to sync to HD with appendonly log, but I really don't want this to happen for the rest of my databases on the same Redis instance (to save disk space and reduce latency).

Redis has a very small memory footprint. This means you can very easily run multiple instances in parallel, where you configure them differently. This will also have a good impact on your latency requirement, because you will have a dedicated instance for your critical data.