Well, this works (appended to my ‘child’ functions.php) for the other thing I wanted:
add_filter( 'generate_blog_get_column_count','tu_search_column_count' );
function tu_search_column_count( $count ) {
if ( is_search() ) {
return 100;
}
return $count;
}
Maybe not the best or only way (?), but it’s the one I’ve got to work!