This is not a needed feature, just a useful thing.
As a matter of fact, Volumio may be for many people the only UNIX computer always-on. It could be useful for someone to use it as entrance from the internet to the home network, even if only for a SSH channel. However, opening a SSH port on the internet is dangerous, there is risk for brute force (and using keys is not comfortable, you need them with you).
I suggest to provide (but not activate) the Google Authenticator PAM module
code.google.com/p/google-authenticator/
It gives the option to use both a password and a one-time-only code to login. This way only if you have both (one of which is also resistant against keyloggers) you can get in.
I asked for it because compiling it is extremely easy if you have the environment, but installing the environment in Volumio takes time and space.