MrMaz has created BP-Links which allows for easy submitting of rich media content. It’s not possible (yet) to use it in the activity stream but he said he would look into it when BP 1.2 is released.
You can find the plugin and more info here;
https://wordpress.org/extend/plugins/buddypress-links/
It does exactly what you say; Fill in a link to youtube/flickr and it detects the video or photo. The newest version of his plugin (not out yet) allows fetching images from ANY site with images on it, so that will be even more awesome!
Killer plugin which will only get better
If no one writes an oEmbed plugin for the BP activity stream in the first few weeks after BP 1.2 is up, I’ll take a look.