Inject a Bitbucket repo into the WordPress updates UI.

pre_set_site_transient_update_plugins.php

<?php

add_filter( 'pre_set_site_transient_update_plugins', array( $this, 'set_plugin_transient' ) );

/**
 * Inject our updates into core's list of updates.
 * 
 * @param  array $transient The existing list of assets that need an update.
 * @return The list of assets that need an update, filtered.
 */
public function set_plugin_transient( $transient ) {

	if( ! is_array( $this -> assets_to_update ) ) { return $transient; }

	foreach( $this -> assets_to_update as $asset ) {

		if( empty( $asset -> transient_key ) ) { continue; }

		if( ! $asset -> transient_content ) { continue; }

		$transient -> response[ $asset -> transient_key ] = $asset -> transient_content;

	}

	return $transient;

}