Hide admin toolbar based on a query string

You should be able to just add the filter inside of a conditional:

<?php if ($_GET['hidetoolbar']) 
{
    add_filter('show_admin_bar', '__return_false');
}
?>

or, since conditionally adding action handlers and filters is sometimes frowned upon, you could add your own function as a filter and then put your conditional inside that:

<?php 
function my_manage_toolbar()
{
    if ($_GET['hidetoolbar']) 
    {
        return false;
    }
    return true;
}
add_filter('show_admin_bar', 'my_manage_toolbar');
?>