If you are thinking about auto-unlocking the root filesystem or any related FS's that are needed for the system startup, then you're obviously right. This is not what I'm referring to though. The containers I'm thinking about are data containers. They can be mounted at will, like the containers that are currently supported by the plugin.
For those, I propose a new feature: configure the name (full path) of a key file, with which the container is encrypted. So long as they key file is absent, the status is as it is now (no unlocking). Once the file becomes available (say, a USB drive has been mounted), the container is auto-unlocked.