Is there a way of registering a default logo image for custom-logo?

Question

Similair to custom-header with register_default_headers() in version 4.5 and above.

Answer 1

No, there isn't.

Given that a logo is typically unique for each site and brand, it doesn't really make much sense to offer a default logo, except as a placeholder.

If you want to have a default logo as a placeholder in your theme, then in the template you could check if a custom value is set, and if not, display your own logo:

<?php
if ( get_custom_logo() ) {
    the_custom_logo();
} else {
    printf(
        '<a href="%1$s" class="custom-logo-link" rel="home" itemprop="url"><img src="%2$s" class="custom-logo" itemprop="logo" alt="%3$s"></a>',
        esc_url( home_url( '/' ) ),
        esc_url( get_theme_file_uri( 'images/default-logo.png' ) ),
        get_bloginfo( 'name', 'display' )
    );
}
?>

The markup in this example will match the markup output of the_custom_logo(). The logo itself would be default-logo.png in the images/ directory of your theme.