Reply to https://buddypress.org/support/topic/custom-post-type-support-for-activity/#post-237701

bp-custom.php

<?php

function job_listing_activity_args() {
	if ( ! bp_is_active( 'activity' ) ) {
		return;
	}

	add_post_type_support( 'job_listing', 'buddypress-activity' );

	bp_activity_set_post_type_tracking_args( 'job_listing', array(
		'component_id'             => 'activity',
		'action_id'                => 'new_job_listing',
        	'bp_activity_admin_filter' => __( 'Published a new Job', 'text-domain' ),
        	'bp_activity_front_filter' => __( 'Job', 'text-domain' ),
        	'contexts'                 => array( 'activity', 'member' ),
        	'activity_comment'         => true,
        	'bp_activity_new_post'     => __( '%1$s posted a new Job: <a href="%2$s">[Job]</a>', 'text-domain' ),
        	'bp_activity_new_post_ms'  => __( '%1$s posted a new Job: <a href="%2$s">[Job]</a>, on the site %3$s', 'text-domain' ),
        	'position'                 => 100,
    	) );
}
add_action( 'init', 'job_listing_activity_args' );


function job_listing_include_post_type_title( $action, $activity ) {
	if ( empty( $activity->id ) ) {
		return $action;
	}

	if ( 'new_job_listing' != $activity->type ) {
		return $action;
	}

	preg_match_all( '/<a.*?>([^>]*)<\/a>/', $action, $matches );

	if ( empty( $matches[1][1] ) || '[Job]' != $matches[1][1] ) {
		return $action;
	}

	$post_type_title = bp_activity_get_meta( $activity->id, 'post_title' );

	if ( empty( $post_type_title ) ) {

		switch_to_blog( $activity->item_id );

		$post_type_title = get_post_field( 'post_title', $activity->secondary_item_id );

		// We have a title save it in activity meta to avoid switching blogs too much
		if ( ! empty( $post_type_title ) ) {
			bp_activity_update_meta( $activity->id, 'post_title', $post_type_title );
		}

		restore_current_blog();
	}

	return str_replace( $matches[1][1], esc_html( $post_type_title ), $action );
}
add_filter( 'bp_activity_custom_post_type_post_action', 'job_listing_include_post_type_title', 10, 2 );

Merci pour le filtre @imath , ça marche comme un charme et c'est très intéressant !