@anonymous-piwik-user i do this in intranet over a custom variable (username or mail) and LDAP.
How do you want to set the avatar?

Already possible hook: Gravatar over custom var mail

as @ThaDafinser indicates, the gravatar can already be customised by a plugin using the hook: http://developer.piwik.org/api-reference/events#livegetextravisitordetails

let us know if you have more question!