YouTube's "Watch Later" feature would be a lot more useful to me if it could include videos that are not hosted on YouTube at all. But that's not going to happen, because (1) it's hard to do and (2) it would mean I'm not watching everything on YouTube. Right now, I'd settle for being able to add any embedded video to my Watch Later list without having to start playing it first.
Mokalus of Borg
PS - It would also, in all likelihood, violate the terms of service of other video sites, too.
PPS - That's assuming you could get it to work at all.