This is possible. For https://mail.clientdomain you can su to the "roundcubelocal" user on the email server and make the change there. If you're changing the logo within the default 'elastic' skin you should make a copy of the skin to ensure the change is not overwritten by updates.
If you're looking to customise the central webmail user you can do the same thing or you can edit it through the file manager in the UI.
