I had faced the same problem.
Wiimc is not able to parse Navi-X processors yet.
To solve that I’ve created a scraper for youtube playlists that create a plx file in which entries are URLs pointing to youtube videos.
At least for youtube, YOU CAN’T HOST the scraper REMOTELY.
Each page impression of youtube, changes by your geographical/network location, i.e. the same youtube video depending in where you are should have a different URL, but you need the right one in order to watch. So the solution
is scrape from your local network.
As I told you I wrote an script that runs on a HTTP server I have in my local network. Then I put an entry in the wiimc that queries the scraper to generate a dynamic created playlist from youtube, and works perfectly.
If you want I can provide you the code. For the HTTP server side configuration, you must know how to do it.