Well, nobody knows a code better than its creator! Your original code assigned to $script_file the value set in <RC root>/plugins/managesieve/config.inc.php ; I forced
$script_file = '/etc/dovecot/sieve/default.sieve';
and indeed, now, when a user with no sieve scripts defined logins to roundcube, he/she sees the default ruleset!
Therefore, either the call $this->rc->config->get('managesieve_default') in managesieve.php doesn't collect any value from managesieve_default, or the assignment $rcmail_config['managesieve_default'] = '/etc/dovecot/sieve/default.sieve'; in managesieve/config.inc.php doesn't assign any value to managesieve_default .
Anyway, for the time being I'm happy with this hack, I'll report whether the problem persists or not as soon as I upgrade Roundcube.
Thanks a lot indeed for your help!!!