BuddyPress.org

hmm, well i was going to suggest using the filter…

in bp_blogs_record_post
'content' => apply_filters( 'bp_blogs_activity_new_post_content', $activity_content, &$post, $post_permalink )

which calls bp_blogs_record_activity but it does not pass the untouched content to tweak to your liking.
$content = apply_filters( 'bp_blogs_record_activity_content', bp_create_excerpt( $content ) ;