I created this function on "print_api.php":

--------------------

# return "TRUE" instead of "selected"
# used by the function "print_project_option_list2()" and "print_subproject_option_list2()"
function check_selected2( $p_var, $p_val=true ) {
    if ( is_array( $p_var ) ) {
        foreach( $p_var as $p_this_var ) {
            if ( $p_this_var == $p_val ) {
                return true;
            }
        }
    } else {
        if ( $p_var == $p_val ) {
            return true;
        }
    }
}

# --------------------
# modified function "print_project_option_list()"
function print_project_option_list2( $p_project_id = null, $p_filter_project_id = null ) {
    project_cache_all();
    $t_project_ids = current_user_get_accessible_projects();
    $t_project_count = count( $t_project_ids );
    for ($i=0;$i<$t_project_count;$i++) {
        $t_id = $t_project_ids[$i];
        if ( $t_id != $p_filter_project_id ) {
            if( check_selected2( $p_project_id, $t_id ) ) {
                PRINT "<option value=\"$t_id\"";
                PRINT '>' . string_display( project_get_field( $t_id, 'name' ) ) . '</option>' . "\n";
            }
            else { # (kivio 29.10.10) Projects that user is assigned shown in color gray
                //PRINT "<option value=\"$t_id\" style='color:gray'";//" disabled='disabled'"
                //PRINT '>' . string_display( project_get_field( $t_id, 'name' ) ) . '</option>' . "\n";
            }// End else
            print_subproject_option_list2( $t_id, $p_project_id, $p_filter_project_id );
        }
    }// End for 
}

# --------------------
# modified function "print_subproject_option_list()"
function print_subproject_option_list2( $p_parent_id, $p_project_id = null, $p_filter_project_id = null, $p_parents = Array() ) {
    array_push( $p_parents, $p_parent_id );
    $t_subproject_ids = current_user_get_accessible_subprojects( $p_parent_id );
    $t_subproject_count = count( $t_subproject_ids );
    for ($j=0;$j<$t_subproject_count;$j++) {
        $t_full_id = $t_id = $t_subproject_ids[$j];
        if ( $t_id != $p_filter_project_id ) {
            if( check_selected2( $p_project_id, $t_full_id ) ) {
                PRINT "<option value=\"";
                PRINT "$t_full_id\"";
                PRINT '>' . str_repeat( "&raquo; ", count( $p_parents ) ) . string_display( project_get_field( $t_id, 'name' ) ) . '</option>' . "\n";
            } 
            else { # (kivio 29.10.10) Subprojects that user is assigned shown in color gray
                //PRINT "<option value=\"";
                //PRINT "$t_full_id\" style='color:gray'";//disabled='disabled'";
                //PRINT '>' . str_repeat( "&raquo; ", count( $p_parents ) ) . string_display( project_get_field( $t_id, 'name' ) ) . '</option>' . "\n";
            }
            print_subproject_option_list2( $t_id, $p_project_id, $p_filter_project_id, $p_parents );
        }
    }// End for
}

# --------------------
# list of projects that a user is NOT in
function print_project_user_list_option_list2( $p_user_id ) {

    $t_mantis_project_user_list_table = config_get( 'mantis_project_user_list_table' );
    $t_mantis_project_table = config_get( 'mantis_project_table' );

    $c_user_id = db_prepare_int( $p_user_id );

    $query = "SELECT DISTINCT p.id, p.name
            FROM $t_mantis_project_table p
            LEFT JOIN $t_mantis_project_user_list_table u
            ON p.id=u.project_id AND u.user_id='$c_user_id'
            WHERE p.enabled=1 AND
                u.user_id IS NULL
            ORDER BY p.name";     

    $result = db_query( $query );
    $category_count = db_num_rows( $result );
    $t_project_id = array();

    for ($i=0;$i<$category_count;$i++) {
        $row = db_fetch_array( $result );
        $t_project_name = string_attribute( $row['name'] );
        $t_project_id[$i] = $row['id'];

    }

    print_project_option_list2( $t_project_id, null );

}

Then i call function print_project_user_list_option_list2 on "manage_user_edit_page". Hope it helps.