One of the plugins I work on, Tasty Pins, offers the ability to add "Pinterest Text" to an image (related #8473).

In the Classic Editor, the user can edit their Pinterest Text in the Media Library:

When the image is inserted into the post, the Pinterest Text is included as a data-pinterest-description data attribute:

Even though all of this interaction is clientside, a plugin can modify the <img> image markup sent to the editor via the image_send_to_editor filter (ref):

/**
 * Append Pinterest Text to markup sent to editor
 *
 * @param string $html The image HTML markup to send.
 * @param int    $id   The attachment id.
 * @return string
 */
public static function filter_image_send_to_editor( $html, $id ) {
	if ( ! empty( $_POST['props']['tp_pinterest_text'] ) ) {
		$html = str_replace( '<img ', '<img data-pin-description="' . esc_attr( wp_unslash( $_POST['props']['tp_pinterest_text'] ) ) . '" ', $html );
	}
	if ( ! empty( $_POST['props']['tp_pinterest_nopin'] ) ) {
		$html = str_replace( '<img ', '<img data-pin-nopin="true" ', $html );
	}
	return $html;
}

What's the best way to achieve equivalent behavior in Gutenberg?

One idea, albeit not necessarily a good one, is to make onSelectImage filterable:

onSelectImage seems to be common across Image, Cover Image, and Gallery Blocks. Other approaches?