Funimation would be great as it's both quite large and every bit as legitimate as Hulu, but it might take a bit of effort; I can't find any rss feeds for the site. They even have episodes of One Piece up very shortly after they air in Japan... unfortunately, about a month ago, someone guessed the filename of the episode and downloaded it before it was listed on Funimation's website. The Japanese studio they license from found out, and caused them to shut down their streaming service while they reworked how they name video files (I think it's a long random-looking string now). Everyone who accessed the episode early had their IPs banned from the site(no idea for how long though). Just something to keep in mind for whoever wants to write the plugin.
I started on a plugin a while ago before the One Piece mess, but didn't get very far; I might give it another shot when I get back from vacation, but it's probably best if someone who knows what they are doing takes a crack at it.