You can use the generate_inside_mobile_header hook:
add_action( 'generate_inside_mobile_header','tu_mobile_header_html' );
function tu_mobile_header_html() { ?>
<div class="mobile-header-content">
Your site title etc..
</div>
<?php }
add_action( 'generate_inside_mobile_header','tu_mobile_header_html' );
function tu_mobile_header_html() { ?>
<div class="mobile-header-content">
Your site title etc..
</div>
<?php }
I also would like the site title and the menu toggle in one line with the site title floating left and the menu toggle button floating right (hamburger menu).